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2022-03-29 | Ab initio Approaches to High Entropy Alloys: A Comparison of CPA, SQS, and Supercell Methods | We present a comparative study of different modeling approaches to the
electronic properties of the
$\textrm{Hf}_{0.05}\textrm{Nb}_{0.05}\textrm{Ta}_{0.8}\textrm{Ti}_{0.05}\textrm{Zr}_{0.05}$
high entropy alloy. Common to our modeling is the methodology to compute the
one-particle Green's function in the framework of density functional theory. We
demonstrate that the special quasi-random structures modeling and the
supercell, i.e. the locally self-consistent multiple-scatering methods provide
very similar results for the ground state properties such as the spectral
function (density of states) and the equilibrium lattice parameter. To
reconcile the multiple-scattering single-site coherent potential approximation
with the real space supercell methods, we included the effect of screening of
the net charges of the alloy components. Based on the analysis of the total
energy and spectral functions computed within the density functional theory, we
found no signature for the long-range or local magnetic moments formation in
the
$\textrm{Hf}_{0.05}\textrm{Nb}_{0.05}\textrm{Ta}_{0.8}\textrm{Ti}_{0.05}\textrm{Zr}_{0.05}$
high entropy alloy, instead we find possible superconductivity below $\sim 9$K. | 2203.16498v1 |
2016-03-24 | Interplay between magnetism and energetics in FeCr alloys from a predictive non-collinear magnetic tight-binding model | Magnetism is a key driving force controlling several thermodynamic and
kinetic properties of Fe-Cr systems. We present a newly-developed TB model for
Fe-Cr, where magnetism is treated beyond the usual collinear approcimation. A
major advantage of this model consists in a rather simple fitting procedure. In
particular, no specific properties of the binary system is explicitly required
in the fitting database. The present model is proved to be accurate and highly
transfer-able for electronic, magnetic and energetic properties of a large
variety of structural and chemical environments: surfaces, interfaces, embedded
clusters, and the whole compositional range of the binary alloy. The occurence
of non-collinear magnetic configurations caused by magnetic frustrations is
successfully predicted. The present TB approach can apply for other binary
magnetic transition-metal alloys. It is expected to be particularly promissing
if the size difference between the alloying elements is rather small and the
electronic properties prevail. | 1603.07482v1 |
2017-02-16 | Extended spin model in atomistic simulations of alloys | An extended atomistic spin model allowing for studies of the finite
temperature magnetic properties of alloys is proposed. The model is obtained by
extending the Heisenberg Hamiltonian via a parameterization from a first
principles basis, interpolating from both the low temperature ferromagnetic and
the high temperature paramagnetic reference states. This allows us to treat
magnetic systems with varying degree of itinerant character within the model.
Satisfactory agreement with both previous theoretical studies and experiments
are obtained in terms of Curie temperatures and paramagnetic properties. The
proposed model is not restricted to elements but is also applied to binary
alloys, such as the technologically important material Permalloy, where
significant differences in the finite magnetic properties of Fe and Ni magnetic
moments are found. The proposed model strives to find the right compromise
between accuracy and computational feasibility for accurate modeling, even for
complex magnetic alloys and compounds. | 1702.05011v1 |
2019-03-20 | Effect of solute content and temperature on the deformation mechanisms and critical resolved shear stress in Mg-Al and Mg-Zn alloys | The influence of solute atoms (Al and Zn) on the deformation mechanisms and
the critical resolved shear stress for basal slip in Mg alloys at 298 K and 373
K was ascertained by micropillar compression tests in combination with
high-throughput processing techniques based on the diffusion couples. It was
found that the presence of solute atoms enhances the size effect at 298 K as
well as the localization of deformation in slip bands, which is associated with
large strain bursts in the resolved shear stress ($\tau_{RSS}$)-strain
($\epsilon$) curves. Deformation in pure Mg and Mg alloys was more homogeneous
at 373 K and the influence of the micropillar size on the critical resolved
shear stress was much smaller. In this latter case, it was possible to
determine the effect of solute content on the critical resolved shear stress
for basal slip in Mg-Al and Mg-Zn alloys. | 1903.08388v1 |
2019-03-22 | Broadband Phonon Scattering in PbTe-based Materials Driven Near the Ferroelectric Phase Transition by Strain or Alloying | The major obstacle in the design of materials with low lattice thermal
conductivity is the difficulty in efficiently scattering phonons across the
entire frequency spectrum. Using first principles calculations, we show that
driving PbTe materials to the brink of the ferroelectric phase transition could
be a powerful strategy to solve this problem. We illustrate this concept by
applying tensile [001] strain to PbTe and its alloys with another rock-salt
IV-VI material, PbSe; and by alloying PbTe with a rhombohedral IV-VI material,
GeTe. This induces extremely soft optical modes at the zone center, which
increase anharmonic acoustic-optical coupling and decrease phonon lifetimes at
all frequencies. We predict that PbTe, Pb(Se,Te) and (Pb,Ge)Te alloys driven
close to the phase transition in the described manner will have considerably
lower lattice thermal conductivity than that of PbTe (by a factor of 2-3). The
proposed concept may open new opportunities for the development of more
efficient thermoelectric materials. | 1903.09674v1 |
2019-05-01 | Lattice-Constant and Band-Gap Tuning in Wurtzite and Zincblende BInGaN Alloys | InGaN light-emitting diodes (LEDs) are more efficient and cost effective than
incandescent and fluorescent lighting, but lattice mismatch limits the
thickness of InGaN layers that can be grown on GaN without
performance-degrading dislocations. In this work, we apply hybrid density
functional theory calculations to investigate the thermodynamic stability,
lattice parameters, and band gaps of wurtzite and zincblende quaternary BInGaN
alloys. We find that the wurtzite phase is more stable and can be
lattice-matched to GaN for BInGaN compositions containing up to ~30% boron. The
lattice match with GaN decreases strain and enables thicker active layers that
mitigate Auger recombination and increase the efficiency of the LEDs. The band
gap of the alloy remains tunable throughout the visible spectrum. Our results
indicate that BInGaN alloys are promising alternatives to InGaN for
high-efficiency, high-power LEDs. | 1905.00467v2 |
2019-05-09 | Homogenous $In_{x}Ga_{1-x}N$ alloys on ZnO substrates: A new approach for high performance thermoelectric materials | High performance thermoelectric materials for wide-range temperature
applications still remains a challenge. In this study, we have produced
high-quality homogeneous $In_{0.32}Ga_{0.68}N$ on ZnO substrates, with no phase
separation at high Indium content, using metal organic chemical vapor
deposition for thermoelectric applications. A record high room temperature
figure of merit zT is obtained of 0.86, which is five times larger than that of
SiGe, the current state of the art high temperature thermoelectric material.
These materials are shown to have a nearly perfect doping concentration to
maximize zT regardless of the scattering mechanism. This almost one order of
magnitude increase in zT is due to large electrical conductivities from oxygen
co-doping as well as low thermal conductivities from alloy scattering. The
maximum power factor reached was $77.98x10^{-4} W/mK^{2}$ at 300K for
$In_{0.32}Ga_{0.68}N$ alloys at a carrier concentration $~6.25x10^{20}
cm^{-3}$. This work indicates that $In_{x}Ga_{1-x}N$ alloys have great
potential for thermoelectric applications especially at a high temperature
range. | 1905.03769v2 |
2019-05-23 | Nanoporous Aluminum-Magnesium Alloy for UV enhanced spectroscopy | We report the first preparation of nanoporous Al-Mg alloy films by selective
dissolution of Mg from a Mg-rich AlxMg1-x alloy. We show how to tune the
stoichiometry, the porosity and the oxide contents in the final film by
modulating the starting ratio between Al and Mg and the dealloying procedure.
The obtained porous metal can be exploited for enhanced UV spectroscopy. In
this respect, we experimentally demonstrate its efficacy in enhancing
fluorescence and surface Raman scattering for excitation wavelengths of 360 nm
and 257 nm respectively. Finally, we numerically show the superior performance
of the nanoporous Al-Mg alloy in the UV range when compared to equivalent
porous gold structures. The large area to surface ratio provided by this
material make it a promising platform for a wide range of applications in
UV/deep-UV plasmonics. | 1905.09489v1 |
2019-08-28 | Solid-state dewetting instability in thermally-stable nanocrystalline binary alloys | Practical applications of nanocrystalline metallic thin films are often
limited by instabilities. In addition to grain growth, the thin film itself can
become unstable and collapse into islands through solid-state dewetting.
Selective alloying can improve nanocrystalline stability, but the impact of
this approach on dewetting is not clear. In this study, two alloys that exhibit
nanocrystalline thermal stability as ball milled powders are evaluated as thin
films. While both alloys demonstrated dewetting behavior following annealing,
the severity decreased in more dilute compositions. Ultimately, a balance may
be struck between nanocrystalline stability and thin film structural stability
by tuning dopant concentration. | 1908.10504v2 |
2019-08-28 | Two channel heat conduction in the superconducting state of the as-cast V$_{1-x}$Zr$_x$ alloys | We present here the temperature dependence of heat capacity ($C$($T$)) and
thermal conductivity ($\kappa$($T$)) in the superconducting state as well as in
the normal state of as-cast V$_{1-x}$Zr$_x$ alloys. Distinct jumps in the
$C$($T$) of the alloys indicate the presence of three superconducting phases
with transition temperatures $T_{C1}$ = 5.4~K, $T_{C2}$ = 8.2~K and $T_{C3}$ =
8.5~K. From the metallography micrographs, these three phases are identified to
be $\beta$-V, $\gamma$-ZrV$_2$, and $\gamma'$-ZrV$_2$ respectively. Apart from
these phases, $\alpha$-Zr and $\beta$-Zr phases are also detected in these
samples. The experimental $\kappa$($T$) in the superconducting state of these
alloys is observed to be significantly higher than that expected theoretically.
Our analysis suggests that the above observation is due to the coexistence of
multiple superconducting and non superconducting phases which resulted in the
two-parallel channels for the conduction of heat. | 1908.10570v1 |
2019-09-11 | A Review on Superplastic Forming of Ti-6Al-4V Alloy | This paper presents a review on the superplastic forming of Ti-6Al-4V alloy,
which has been used to manufacture parts of complex shapes and geometries. This
paper outlines the major work carried out on this front in the past three
decades. It covers various aspects related to experimental setups, including
the manufacture of dies and their modifications to maintain alloy thickness
uniformity after forming. A detailed study of the process parameters has also
been done to note the most important physical conditions required for
successful forming. This is followed by the influence of microstructure, modern
applications of superplastic forming of different titanium alloys and is
concluded with an insight into the future work and progress in this field. | 1909.05011v2 |
2019-09-15 | Effect of gallium doping on bubbling and helium retention in aluminum exposed to low-energy helium plasma | Surface bubbling and helium retention of pure aluminum and a solid solution
of aluminum-gallium(Ga) alloy exposed to low-energy He plasma has been
investigated with the fluence of 1.8E24 He/m2 at room temperature . Surface
morphology observations show that after irradiation, the average size of He
bubbles on Al-1.8 at.% Ga alloy is around 1.8 micro metre, smaller than that on
pure Al. Ga doping noticeably increases the area density of the bubble. The
thermal desorption spectroscopyshows that the release amount of He in Al-1.8
at.% Ga alloy is 1.77E21 He/m2, nearly three orders of magnitude higher than
that in pure Al, whlie the He desorption peak of Al-1.8 at.% Ga alloy is 480 K,
much lower than 580 K of pure Al. The results of slow positron annihilation
spectroscopy (SPAS) indicate that the vacancy type defects were introduced by
He plasma irradiation; and lattice distortion caused by Ga doping plays an
important role in determining surface bubbling and He retention characteristics
of Al-1.8 at.% Ga. | 1909.06815v1 |
2019-12-23 | Change of electronic properties on transition from high-entropy to Ni-rich (TiZrNbCu)(1-x)Ni(x) alloys | We present results of comprehensive study of electronic properties of
(TiZrNbCu)(1-x)Ni(x) metallic glasses performed in broad composition range x
encompassing both, high entropy (HE) range, and conventional Ni-base alloy
concentration range, x >= 0.35. The electronic structure studied by
photoemission spectroscopy and low temperature specific heat (LTSH) reveal a
split-band structure of density of states inside valence band with d-electrons
of Ti, Zr, Nb and also Ni present at Fermi level N(E_F), whereas LTSH and
magnetoresistivity results show that variation of N(E_F) with x changes in
Ni-base regime. The variation of superconducting transition temperatures with x
closely follows that of N(E_F). The electrical resistivities of all alloys are
high and decrease with increasing temperature over most of explored temperature
range, and their temperature dependence seems dominated by weak localization
effects over a broad temperature range (10-300 K). The preliminary study of
Hall effect shows positive Hall coefficient that decreases rapidly in Ni-base
alloys. | 1912.11133v1 |
2020-02-03 | New approach for FIB-preparation of atom probe specimens for aluminum alloys | Site-specific atom probe tomography (APT) from aluminum alloys has been
limited by sample preparation issues. Indeed, Ga, which is conventionally used
in focused-ion beam (FIB) preparations, has a high affinity for Al grain
boundaries and causes their embrittlement. This leads to high concentrations of
Ga at grain boundaries after specimen preparation, unreliable compositional
analyses and low specimen yield. Here, to tackle this problem, we propose to
use cryo-FIB for APT specimen preparation specifically from grain boundaries in
a commercial Al-alloy. We demonstrate how this setup, easily implementable on
conventional Ga-FIB instruments, is efficient to prevent Ga diffusion to grain
boundaries. Specimens were prepared at room temperature and at cryogenic
temperature (below approx. 90K) are compared, and we confirm that at room
temperature, a compositional enrichment above 15 at.% of Ga is found at the
grain boundary, whereas no enrichment could be detected for the cryo-prepared
sample. We propose that this is due to the decrease of the diffusion rate of Ga
at low temperature. The present results could have a high impact on the
understanding of aluminum and Al-alloys. | 2002.00992v1 |
2020-02-08 | Multiscale modelling of precipitation hardening in Al-Cu alloys: dislocation dynamics simulations and experimental validation | The mechanisms of dislocation/precipitate interactions were analyzed in an
Al-Cu alloy containing a homogeneous dispersion of $\theta'$ precipitates by
means of discrete dislocation dynamics simulations. The simulations were
carried out within the framework of the discrete-continuous method and the
precipitates were assumed to be impenetrable by dislocations. The main
parameters that determine the dislocation/precipitate interactions (elastic
mismatch, stress-free transformation strains, dislocation mobility and
cross-slip rate) were obtained from atomistic simulations, while the size,
shape, spatial distribution and volume fraction of the precipitates were
obtained from transmission electron microscopy. The predictions of the critical
resolved shear stress (including the contribution of solid solution) were in
agreement with the experimental results obtained by means of compression tests
in micropillars of the Al-Cu alloy oriented for single slip. The simulations
revealed that the most important contribution to the precipitation hardening of
the alloy was provided by the stress-free transformation strains followed by
the solution hardening and the Orowan mechanism due to the bow-out of the
dislocations around the precipitates. | 2002.03128v1 |
2020-02-14 | Designing transformation-induced plasticity and twinning-induced plasticity Cr-Co-Ni medium entropy alloys: theory and experiment | In order to efficiently explore the nearly infinite composition space in
multicomponent solid solution alloys, it is important to establish predictive
design strategies and use computation-aided methods. In the present work, we
demonstrated the density functional theory calculations informed design routes
for realizing transformation-induced plasticity (TRIP) and twinning-induced
plasticity (TWIP) in Cr-Co-Ni medium entropy alloys (MEAs). We systematically
studied the effects of magnetism and chemical composition on the generalized
stacking fault energy surface (gamma-surface) and showed that both chemistry
and the coupled magnetic state strongly affect the gamma-surface, consequently,
the primary deformation modes. Based on the calculated effective energy
barriers for the competing deformation modes, we constructed composition and
magnetism dependent deformation maps at both room and cryogenic temperatures.
Accordingly, we proposed various design routes for achieving desired primary
deformation modes in the ternary Cr-Co-Ni alloys. The deformation mechanisms
predicted by our theoretical models are in nice agreement with available
experimental observations in literature. Furthermore, we fabricated two
non-equiatomic Cr-Co-Ni MEAs possessing the designed TWIP and TRIP effects,
showing excellent combinations of tensile strength and ductility. | 2002.05900v1 |
2020-06-01 | Multifunctional behavior of Mn-site doped antiferromagnetic Mn$_5$Si$_3$ alloys | Present work reports a detailed investigation on the magnetoresistance and
magnetocaloric behavior of Ni and Cr-doped Mn$_5$Si$_3$ alloys with general
formula Mn$_{5-x}$A$_x$Si$_3$ (where A = Ni/Cr; $x$ = 0, 0.05, 0.1 and 0.2).
Both pure (undoped) and doped alloys show a reasonably large amount of
magnetoresistance (MR). Doping at Mn-site, both by Ni and Cr, results in a
monotonic decrease in MR values. Magnetocaloric effect (MCE), on the other
hand, is found to be interesting, and all the alloys show both conventional and
inverse MCE around the magneto-structural transition temperature. Among the two
types of MCE observed, the inverse MCE is found to decrease with increasing
doping concentration and consistent with the MR behavior, whereas doping
results in a significant increase in conventional MCE values. | 2006.00865v1 |
2020-06-11 | Low thermal conductivity of iron-silicon alloys at Earth core conditions with implications for the geodynamo | Earth core is composed of iron (Fe) alloyed with light elements, e.g.,
silicon (Si). Its thermal conductivity critically affects Earth thermal
structure, evolution, and dynamics, as it controls the magnitude of thermal and
compositional sources required to sustain a geodynamo over Earth history. Here
we directly measured thermal conductivities of solid Fe and Fe-Si alloys up to
144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its
conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at%
Si would have a conductivity of about 20 W m-1 K-1, lower than pure Fe at
similar pressure-temperature conditions. This suggests a lower minimum heat
flow, around 3 TW, across the core-mantle boundary than previously expected,
and thus less thermal energy needed to operate the geodynamo. Our results
provide key constraints on inner core age that could be older than two
billion-years. | 2006.06271v2 |
2020-06-11 | Relationship between grain boundary segregation and grain boundary diffusion in Cu-Ag alloys | While it is known that alloy components can segregate to grain boundaries
(GBs), and that the atomic mobility in GBs greatly exceeds the atomic mobility
in the lattice, little is known about the effect of GB segregation on GB
diffusion. Atomistic computer simulations offer a means of gaining insights
into the segregation-diffusion relationship by computing the GB diffusion
coefficients of the alloy components as a function of their segregated amounts.
In such simulations, thermodynamically equilibrium GB segregation is prepared
by a semi-grand canonical Monte Carlo method, followed by calculation of the
diffusion coefficients of all alloy components by molecular dynamics. As a
demonstration, the proposed methodology is applied to a GB is the Cu-Ag system.
The GB diffusivities obtained exhibit non-trivial composition dependencies that
can be explained by site blocking, site competition, and the onset of GB
disordering due to the premelting effect. | 2006.06591v2 |
2020-07-30 | Control of hot-carrier relaxation time in Au-Ag thin films through alloying | The plasmon resonance of a structure is primarily dictated by its optical
properties and geometry, which can be modified to enable hot-carrier
photodetectors with superior performance. Recently, metal-alloys have played a
prominent role in tuning the resonance of plasmonic structures through chemical
composition engineering. However, it has been unclear how alloying modifies the
time dynamics of generated hot-carriers. In this work, we elucidate the role of
chemical composition on the relaxation time of hot-carriers for the archetypal
Aux Ag1-x thin-film system. Through time-resolved optical spectroscopy
measurements in the visible wavelength range, we measure composition-dependent
relaxation times that vary up to 8x for constant pump fluency. Surprisingly, we
find that the addition of 2% of Ag into Au films can increase the hot carrier
lifetime by approximately 35% under fixed fluence, as a result of a decrease in
optical loss. Further, the relaxation time is found to be inversely
proportional to the imaginary part of the permittivity. Our results indicate
that alloying is a promising approach to effectively control hot-carrier
relaxation time in metals. | 2007.15561v1 |
2020-09-23 | Indium gallium nitride quantum dots: Consequence of random alloy fluctuations for polarization entangled photon emission | We analyze the potential of the $c$-plane InGaN/GaN quantum dots for
polarization entangled photon emission by means of an atomistic many-body
framework. Special attention is paid to the impact of random alloy fluctuations
on the excitonic fine structure and the excitonic binding energy. Our
calculations show that $c$-plane InGaN/GaN quantum dots are ideal candidates
for high temperature entangled photon emission as long as the underlying
$C_{3v}$-symmetry is preserved. However, when assuming random alloy
fluctuations in the dot, our atomistic calculations reveal that while the large
excitonic binding energies are only slightly affected, the $C_{3v}$ symmetry is
basically lost due to the alloy fluctuations. We find that this loss in
symmetry significantly impacts the excitonic fine structure. The observed
changes in fine structure and the accompanied light polarization
characteristics have a detrimental effect for polarization entangled photon
pair emission via the biexciton-exciton cascade. Here, we also discuss possible
alternative schemes that benefit from the large excitonic binding energies, to
enable non-classical light emission from $c$-plane InGaN/GaN quantum dots at
elevated temperatures. | 2009.11161v1 |
2020-09-30 | Lattice Thermal Transport in Two-Dimensional Alloys and Fractal Heterostructures | Engineering thermal transport in two dimensional materials, alloys and
heterostructures is critical for the design of next-generation flexible
optoelectronic and energy harvesting devices. Direct experimental
characterization of lattice thermal conductivity in these ultra-thin systems is
challenging and the impact of dopant atoms and hetero-phase interfaces,
introduced unintentionally during synthesis or as part of deliberate material
design, on thermal transport properties is not understood. Here, we use
non-equilibrium molecular dynamics simulations to calculate lattice thermal
conductivity of (Mo|W)Se$_2$ monolayer crystals including Mo$_{1-x}$W$_x$Se$_2$
alloys with substitutional point defects, periodic MoSe$_2$|WSe$_2$
heterostructures with characteristic length scales and scale-free fractal
MoSe$_2$|WSe$_2$ heterostructures. Each of these features has a distinct effect
on phonon propagation in the crystal, which can be used to design fractal and
periodic alloy structures with highly tunable thermal conductivities. This
control over lattice thermal conductivity will enable applications ranging from
thermal barriers to thermoelectrics. | 2009.14508v1 |
2020-10-03 | Interstitial Carbon in bcc HfNbTiVZr high entropy alloy from first principles | The remarkable mechanical properties of high entropy alloys can be further
improved by interstitial alloying. In this work we employ density functional
theory calculations to study the solution energies of dilute carbon
interstitial atoms in tetrahedral and octahedral sites in bcc HfNbTiVZr. Our
results indicate that carbon interstitials in tetrahedral sites are unstable,
and the preferred octahedral sites present a large spread in the energy of
solution. The inclusion of carbon interstitials induces large structural
relaxations with long-range effects. The effect of local chemical environment
on the energy of solution is investigated by performing a local cluster
expansion including studies of its correlation with the carbon atomic Voronoi
volume. However, the spread in solution energetics can not be explained with a
local environment analysis only pointing towards a complex, long-range
influence of interstitial carbon in this alloy. | 2010.01354v1 |
2021-01-29 | High temperature interaction between molten Ni50Al50 alloy and ZrB2 ultra-high temperature ceramics | In this work, Ni50Al50 alloy is taken into consideration as potential brazing
material for joining ZrB2 ultra-high temperature ceramic. The results of
experimental study on high temperature interfacial phenomena between molten
binary Ni50Al50 alloy and polycrystalline ZrB2, are shown. A sessile drop
method combined with a capillary purification procedure was applied to
investigate the wetting behavior of Ni50Al50/ZrB2 couple during holding for 400
seconds at temperature of 1688{\deg}C (i.e. at T=1.02Tm). It was found that the
molten Ni50Al50 rapidly wets and spreads over the surface of ZrB2, while
involved reactive infiltration into the solid substrate allowed reaching a
final contact angle of ~0{\deg} in 250 sec. The wetting kinetics was much
faster than that reported in the literature for Cu, Ag or Au tested at
T=1.05Tm. The solidified couple was subjected to SEM/EDS microstructural
characterization in order to reveal a course of interfacial phenomena. The
results point towards (I) an existence of Ni-enriched Ni-Al/ZrB2 surface
interfacial layer; (II) a formation of infiltration zone assisted by reactively
formed Al2O3 due to a reaction with Al-rich melt and (III) a partial transfer
of ZrB2 phase to Ni50Al50 alloy by a dissolution/precipitation mechanism. | 2101.12510v1 |
2021-04-16 | Bandgap widening and behavior of Raman-active phonon modes of cubic single-crystalline (In,Ga)$_2$O$_3$ alloy films | The influence of Ga incorporation into cubic In$_2$O$_3$ on the electronic
and vibrational properties is discussed for (In$_{1-x}$,Ga$_x$)$_2$O$_3$ alloy
films grown by molecular beam epitaxy. Using spectroscopic ellipsometry, a
linear dependence of the absorption onset on the Ga content $x$ is found with a
blueshift of up to 150 meV for $x = 0.1$. Consistently, the fundamental band
gap exhibits a blueshift as determined by hard X-ray photoelectron
spectroscopy. The dependence of the absorption onset and the effective electron
mass on the electron concentration is derived from the infrared dielectric
functions for a Sn doped alloy film. The influence of alloying on phonon modes
is analyzed on the basis of Raman spectroscopic measurements. The frequencies
of several phonon modes are identified as sensitive measures for the
spectroscopic determination of the Ga content. | 2104.08092v2 |
2021-05-05 | From pseudo-direct hexagonal germanium to direct silicon-germanium alloys | We present ab initio calculations of the electronic and optical properties of
hexagonal SiGe alloys in the lonsdaleite structure. Lattice constants and
electronic band structures in excellent agreement with experiment are obtained
using density-functional theory. Hexagonal Si has an indirect band gap, while
hexagonal Ge has a pseudo-direct gap, i.e. the optical transitions at the
minimum direct band gap are very weak. The pseudo-direct character of pure
hexagonal Ge is efficiently lifted by alloying. Already for a small admixture
of Si, symmetry reduction enhances the oscillator strength of the lowest direct
optical transitions. The band gap is direct for a Si content below 45 %. We
validate lonsdaleite group-IV alloys to be efficient optical emitters, suitable
for integrated optoelectronic applications. | 2105.01980v1 |
2021-05-07 | Atomic Origins of Friction Reduction in Metal Alloys | We present the results of large scale molecular dynamics simulations aimed at
understanding the origins of high friction coefficients in pure metals, and
their concomitant reduction in alloys and composites. We utilize a series of
targeted simulations to demonstrate that different slip mechanisms are active
in the two systems, leading to differing frictional behavior. Specifically, we
show that in pure metals, sliding occurs along the crystallographic slip
planes, whereas in alloys shear is accommodated by grain boundaries. In pure
metals, there is significant grain growth induced by the applied shear stress
and the slip planes are commensurate contacts with high friction. However, the
presence of dissimilar atoms in alloys suppresses grain growth and stabilizes
grain boundaries, leading to low friction via grain boundary sliding. | 2105.03054v1 |
2021-08-08 | Atomic structure, electronic structure and optical absorption of inorganic perovskite compounds Cs2SnI6-nXn (X=F, Cl, Br; n= 0~6): A first-principles study | As a possible alternative to organic-inorganic hybrid perovskite halide,
inorganic Cs2SnI6 has drawn more and more research attention recently. In order
to find more Cs2SnI6 derivatives as the potential solar cell absorber
materials, I- ions in Cs2SnI6 are replaced by other halogen ions and forms the
Cs2SnI6-nXn (X=F, Cl, Br; n=1~6) compounds, whose atomic structures, electronic
structures and optical absorption are investigated by first principles
calculation. When the alloying level n increases, the mean lattice constants,
the weighted Sn-X and Cs-X bond lengths all decreases linearly; the bond length
of each Sn-X diminishes slightly inside the octahedral structure; Eg of
Cs2SnI6-nXn increases nonlinearly. Eleven Cs2SnI6-nXn compounds have an Eg
between 1.0 eV and 2.0 eV and so can be potentially used as the light
absorption layer of solar cells. Their partial DOS demonstrate that as the
alloying level n increases, I 5p orbital in VBM and CBM is gradually
substituted by Br 4p, or Cl 3p, or F 2p orbital. The eleven Cs2SnI6-nXn alloys
all have a direct bandgap although the lattice distortion induced by the
alloyed X- ion. | 2108.03597v1 |
2021-08-26 | An investigation of high entropy alloy conductivity using first-principles calculations | The Kubo-Greenwood equation, in combination with the first-principles
Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) can be used
to calculate the DC residual resistivity of random alloys at T = 0 K. We
implemented this method in a multiple scattering theory based ab initio
package, MuST, and applied it to the ab initio study of the residual
resistivity of the high entropy alloy Al$_x$CoCrFeNi as a function of $x$. The
calculated resistivities are compared with experimental data. We also predict
the residual resistivity of refractory high entropy alloy MoNbTaV$_x$W. The
calculated resistivity trends are also explained using theoretical arguments. | 2108.11739v1 |
2021-09-16 | Pinning of dislocations in disordered alloys: Effects of dislocation orientation | The current interest in compositionally complex alloys including so called
high entropy alloys has caused renewed interest in the general problem of
solute hardening. It has been suggested that this problem can be addressed by
treating the alloy as an effective medium containing a random distribution of
dilatation and compression centers representing the volumetric misfit of atoms
of different species. The mean square stresses arising from such a random
distribution can be calculated analytically, their spatial correlations are
strongly anisotropic and exhibit long-range tails with third-order power law
decay. Here we discuss implications of the anisotropic and long-range nature of
the correlation functions for the pinning of dislocations of arbitrary
orientation. While edge dislocations are found to follow the standard pinning
paradigm, for dislocations of near screw orientation we demonstrate the
co-existence of two types of pinning energy minima. | 2109.07796v1 |
2021-10-04 | Spatial characteristics of nickel-titanium shape memory alloy fabricated by continuous directed energy deposition | Additive manufacturing has been adopted to process nickel-titanium shape
memory alloys due to its advantages of flexibility and minimal defects. The
current layer-by-layer method is accompanied by a complex temperature history,
which is not beneficial to the final characteristics of shape memory alloys. In
this study, a continuous directed energy deposition method has been proposed to
improve microstructure uniformity. The spatial characterization of
nickel-titanium shape memory alloy fabricated by continuous directed energy
deposition is investigated to study the temperature history, phase constituent,
microstructure, and mechanical properties. The results indicate that the
fabricated specimen has a monotonic temperature history, relatively uniform
phase distribution and microstructure morphology, as well as high compressive
strength (2982 MPa~3105 MPa) and strain (37.7%~41.1%). The reported method is
expected to lay the foundation for spatial control during the printing of
functional structures. | 2110.01144v1 |
2021-10-08 | Defect studies in strain-relaxed Si$_{1-x}$Ge$_x$ alloys | Raman light scattering, low-temperature photoluminescence, light-scattering
tomography, and hydrogenation were used to investigate optical properties of
defects in strain-relaxed Si_{1-x}Ge_x (0.05 \le x \le 0.50) alloys. The
photoluminescence emission was characterized by typical zero-phonon,
phonon-assisted, and dislocations-related emissions, which are dependent on Ge
composition x. However, luminescence spectra exhibited above band-gap features,
which are likely associated with the presence of Si-rich regions in the alloys.
The results are correlated with light-scattering tomography, revealing the
presence of dislocations and Si precipitates. The excess peak at 519 cm^{-1} in
Ge-rich samples is supportive of this observation. At low Ge content, a
dislocation-related band (D2 line) at 14,204 {\AA} dominates D-band emission
for x < 0.25 while overall D-band emission intensity decreases with x.
Hydrogenation was found to enhance D-band emission, indicating a passivation of
nonradiative recombination centers inside dislocation cores. Si-Si, Si-Ge, and
Ge-Ge phonons (TO, TA, and LA), which are participating in luminescence
emission, evolve with increasing Ge content and Ge-Ge and Si-Ge TO lines
dominate the Raman spectrum to the detriment of the Si-Si TO phonon line. Raman
spectra reveal the presence of alloy fluctuations and possible presence of Ge
particles, particularly in Ge-rich samples. | 2110.04351v1 |
2021-10-12 | Strong paramagnetic response in Y containing V$_{0.6}$Ti$_{0.4}$ superconductor | We report here, the systematic field-cooled (FC) magnetisation of
superconducting (V$_{0.6}$Ti$_{0.4}$)-Y alloys in presence of applied magnetic
fields upto 7 T. Paramagnetic response is clearly observed just below the
superconducting transition temperature (T$_{c}$) in low fields ($\leq$0.2 T).
The lower T$_{c}$ of the Y-rich precipitates as compared to the bulk, is the
origin of flux compression and this leads to paramagnetic response. It is also
observed that the magnetisation obtained during field cooled (FC) cooling cycle
is lower than that of FC warming, for all the alloys in the field range 0.02-7
T. In addition, paramagnetic relaxation of FC moment is observed. We identify
that these features of Y containing alloys are related to the high field
paramagnetic Meissner effect (HFPME). Our analysis shows that the large
difference in pinning strength of the different pinning centres generated due
to Y addition to V$_{0.6}$Ti$_{0.4}$ alloy, is responsible for the observed
effect. We provide further evidence to our claim in the form of extension of
range in temperature and magnetic fields over which HFPME is observed when
samples are subjected to cold work. | 2110.05921v2 |
2021-10-27 | Nanocrystalline FeCr alloys synthesised by severe plastic deformation -- a potential material for exchange bias and enhanced magnetostriction | This work gives insights into processing and characterisation of bulk
nanocrystalline FeCr materials. The investigated FeCr alloys, consisting of 30,
50 and 70 at.% ferromagnetic Fe and remaining anti-ferromagnetic Cr, are
processed by arc melting and subsequent severe plastic deformation by high
pressure torsion. The physical similarities between elemental Fe and Cr in
combination with the nanocrystalline structure of the as-deformed alloys,
necessitates advanced characterisation techniques for the as-deformed state:
In-situ annealing synchrotron X-ray diffraction measurements as well as
electron microscopy experiments are linked to magnetostrictive measurements and
reveal a single phase microstructure. Surprisingly, the nanocrystalline FeCr
alloys remain supersaturated solid solutions upon annealing above 500{\deg}C,
meaning a decomposition in a FeCr nanocomposite is suppressed. For the chosen
annealing conditions grain growth is faster than decomposition and enhanced
magnetostrictive values are found compared to materials in the as-deformed
state. | 2110.14303v1 |
2021-10-31 | Edge dislocations in multi-component solid solution alloys: Beyond traditional elastic depinning | High-entropy alloys (HEA) form solid solutions with large chemical disorder
and excellent mechanical properties. We investigate the origin of HEA
strengthening in face-centered cubic (FCC) single-phase HEAs through molecular
dynamics simulations of dislocations, in particular, the equiatomic $\rm
CrCoNi$, $\rm CrMnCoNi$, $\rm CrFeCoNi$, $\rm CrMnFeCoNi$, $\rm FeNi$, and
also, $\rm Fe_{0.4}Mn_{0.27}Ni_{0.26}Co_{0.05}Cr_{0.02}$, $\rm
Fe_{0.7}Ni_{0.11}Cr_{0.19}$. The dislocation correlation length $\xi$,
roughness amplitude $R_{a}$, and stacking fault widths $W_{SF}$ are tracked as
a function of stress. All alloys are characterized by a well defined depinning
stress ($\sigma_c$) and we find a novel regime where exceptional strength is
observed, and a fortuitous combination takes place, of small stacking fault
widths and large dislocation roughness $R_{a}$. Thus the depinning of two
partials seems analogous to unconventional domain wall depinning in disordered
magnetic thin films. This novel regime is identified in specific compositions
commonly associated with exceptional mechanical properties ($\rm CrCoNi$, $\rm
CrMnCoNi$, $\rm CrFeCoNi$, and $\rm CrMnFeCoNi$). Yield stress from analytical
solute-strengthening models underestimates largely the results in these cases.
A possible strategy for increasing strength in multi-component single-phase
alloys is the combined design of stacking fault width and element-based
chemical disorder. | 2111.00568v1 |
2021-11-08 | Biaxial Strain Modulated Valence Band Engineering in III-V Digital Alloys | Some III-V digital alloy avalanche photodiodes exhibit low excess noise.
These alloys have low hole ionization coefficients due to presence of small
'minigaps', enhanced effective mass and large separation between light-hole and
split-off bands in the valence band. In this letter, an explanation for the
formation of the minigaps using a tight binding picture is provided.
Furthermore, we demonstrate that decreasing substrate lattice constant can
increase the minigap size and mass in the transport direction. This leads to
reduced quantum tunneling and phonon scattering of the holes. Finally, we
illustrate the band structure modification with substrate lattice constant for
other III-V digital alloys. | 2111.04247v1 |
2021-11-22 | Multihyperuniform Long-Range Order in Medium-Entropy Alloys | We provide strong numerical evidence for a hidden multihyperuniform
long-range order (MHLRO) in SiGeSn medium-entropy alloys (MEAs), in which the
normalized infinite-wavelength composition fluctuations for all three atomic
species are completely suppressed as in a perfect crystalline state. We show
this MHLRO naturally leads to the emergence of short-range order (SRO) recently
discovered in MEAs, which results in stable lower-energy states compared to
alloy models with random or special quasi-random structures (SQSs) possessing
no atomic SROs. The MHLRO MEAs approximately realize the Vegard's law, which
offers a rule-of-mixture type predictions of the lattice constants and
electronic band gap, and thus can be considered as an ideal mixing state. The
MHLRO also directly gives rise to enhanced electronic band gaps and superior
thermal transport properties at low temperatures compared to random structures
and SQSs, which open up novel potential applications in optoelectronics and
thermoelectrics. Our analysis of the SiGeSn system leads to the formulation of
general organizing principles applicable in other medium- and high-entropy
alloys (HEAs), and a highly efficient computational model for rendering
realistic large-scale configurations of MEAs and HEAs. | 2111.11412v1 |
2022-01-10 | Phase Boundary Segregation in Multicomponent Alloys: A Diffuse-Interface Thermodynamic Model | Microalloying elements tend to segregate to the matrix-precipitate phase
boundaries to reduce the interfacial energy. The segregation mechanism is
emerging as a novel design strategy for developing precipitation-hardened
alloys with significantly improved coarsening resistance for high temperature
applications. In this paper, we report a nanoscopic diffuse-interface
thermodynamic model that describes multicomponent segregation behavior in
two-phase substitutional alloys. Following classical approaches for grain
boundaries, we employ the regular solution thermodynamics to establish
segregation isotherms. We show that the model recovers the Guttmann
multicomponent isotherm describing local interfacial concentrations, and the
generalized Gibbs adsorption isotherm that governs the total solute excess and
interfacial energy. A variety of multicomponent segregation behaviors are
demonstrated for a model two-phase quaternary alloy. The nature of interfacial
parameters and the resulting analytic solutions make the model amenable for
parameterization and comparison with atomistic calculations and experimental
characterizations. | 2201.03117v1 |
2022-01-15 | The Properties of doping ZrCo | ZrCo alloy is promising to substitute uranium for handling hydrogen isotope
storage in thermonuclear reactor. The alloying substitution of Zr in ZrCo with
Hf or Ti can enhance the ability of anti-disproportionation. In this work,
Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co were considered in the
framework of density functional theory, aimed to investigate the properties of
the alloying substitution of Zr with Hf or Ti in ZrCo. Our results found that
the optimized lattice constants of the alloying substitutions,
Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co, are smaller than ZrCo.
The thermodynamic stability reduces in the order Zr$_{0.75}$Hf$_{0.25}$Co >
ZrCo > Zr$_{0.75}$Ti$_{0.25}$Co, as demonstrated by the enthalpy of formation.
The valence electrons are mainly localized at the ion core and the chemical
bonds are polarized in Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co
analogous to ZrCo. | 2201.05805v1 |
2022-01-19 | Visualization of Band Shifting and Interlayer Coupling in WxMo1-xS2 Alloys using Near-Field Broadband Absorption Microscopy | Beyond-diffraction-limit optical absorption spectroscopy provides profound
information on the graded band structures of composition-spread and stacked
two-dimensional materials, in which direct/indirect bandgap, interlayer
coupling, sliding, and possible defects significantly modify their
optoelectronic functionalities such as photoluminescence efficiency. We here
visualize the spatially-varying band structure of monolayer and bilayer
transition metal dichalcogenide alloys for the first time by using near-field
broadband absorption microscopy. The near-field-spectral and -spatial diagrams
manifest the excitonic band shift that results from the interplay of
composition spreading and interlayer coupling. These results enable us to
identify the top layer of the bilayer alloy as pure WS2. We also use the
aberration-free near-field transmittance images to demarcate the exact
boundaries of alloyed and pure transition metal dichalcogenides. This
technology can offer new insights on various layered structures in the era of
stacking science in quest of novel quantum optoelectronic devices. | 2201.07732v1 |
2022-04-04 | Efficient machine-learning model for fast assessment of elastic properties of high-entropy alloys | We combined descriptor-based analytical models for stiffness-matrix and
elastic-moduli with mean-field methods to accelerate assessment of
technologically useful properties of high-entropy alloys, such as strength and
ductility. Model training for elastic properties uses Sure-Independence
Screening (SIS) and Sparsifying Operator (SO) method yielding an optimal
analytical model, constructed with meaningful atomic features to predict target
properties. Computationally inexpensive analytical descriptors were trained
using a database of the elastic properties determined from density functional
theory for binary and ternary subsets of Nb-Mo-Ta-W-V refractory alloys. The
optimal Elastic-SISSO models, extracted from an exponentially large feature
space, give an extremely accurate prediction of target properties, similar to
or better than other models, with some verified from existing experiments. We
also show that electronegativity variance and elastic-moduli can directly
predict trends in ductility and yield strength of refractory HEAs, and reveals
promising alloy concentration regions. | 2204.01788v1 |
2022-04-09 | Electrical breakdown of a dielectric for the formation of a superconducting nanocontact | Electrical breakdown of the dielectric nanolayer between film electrodes of
niobium and an alloy of 50% indium and 50% tin forms a bridge of this alloy
between the electrodes. The bridge resistance depends on the breakdown current.
The length of the bridge is equal to the thickness of the dielectric (30 nm),
and its diameter is 25 nm. The calculated coherence length of the alloy at 0 K
is close to the length of the bridge. The calculated critical current of a
bridge with a resistance of 1 {\Omega} at a temperature of 0 K is 2 mA. It is
concluded that such a bridge should have the properties of a Josephson contact
at a temperature lower than the critical temperature of the alloy (6.5 K). | 2204.04536v1 |
2022-05-04 | Hole Mobility Calculation for Monolayer Molybdenum Tungsten Alloy Disulfide | A simple band model using higher order non-parabolic effect was adopted for
single layer molybdenum tungsten alloy disulfide (i.e.,
$\mathrm{Mo}_{1-x}\mathrm{W}_x\mathrm{S}_2$). The first-principles method
considering $2\times2$ supercell was used to study band structure of single
layer alloy $\mathrm{Mo}_{1-x}\mathrm{W}_x\mathrm{S}_2$ and a simple band
(i.e., effective mass approximation model, EMA) model with higher order
non-parabolic effect was used to fit the first-principle band structures in
order to calculate corresponding the hole mobility. In addition, we investigate
the alloy scattering effect on the hole mobility of
$\mathrm{Mo}_{1-x}\mathrm{W}_x\mathrm{S}_2$. | 2205.02185v1 |
2022-05-23 | Maximum strength and dislocation patterning in multi-principal element alloys | Multi-principal element alloys (MPEAs), commonly termed as medium- or
high-entropy alloys containing three or more components in high concentrations,
render a tunable chemical short-range order (SRO). Leveraging large-scale
atomistic simulations, we probe the limit of Hall-Petch strengthening and
deformation mechanisms in a model CrCoNi alloy and unravel chemical short-range
ordering effects. It is found that, in the presence of SRO, the maximum
strength is appreciably increased, and the strongest grain size drifts to a
small value. Additionally, the propensity for faulting and deformation
transformation is reduced and accompanied by the intensification of planar slip
and strain localization. We reveal strikingly different deformation
microstructures and dislocation patterns that prominently depend on
crystallographic grain orientation and the number of slip planes activated
during deformation. Grain of single planar slip attains the highest volume
fraction of deformation-induced structure transformation, and grain with double
active slip planes develops the densest dislocation network. These results
advancing the fundamental understanding of deformation mechanisms and
dislocation patterning in MPEAs suggest a mechanistic strategy for tuning
mechanical behavior through simultaneously tailoring grain texture and local
chemical order. | 2205.11053v1 |
2022-06-04 | Chemical Short-Range Ordering in a CrCoNi Medium-Entropy Alloy | The exceptional mechanical strengths of medium and high-entropy alloys have
been attributed to hardening in random solid solutions. Here, we evidence
non-random chemical mixings in CrCoNi alloys, resulting from short range
ordering. A novel data-mining approach of electron nanodiffraction patterns
enabled the study, which is assisted by neutron scattering, atom probe
tomography, and diffraction simulation using first principles theory models.
Results reveal two critical types of short range orders in nanoclusters that
minimize the Cr and Cr nearest neighbors (L11) or segregate Cr on alternating
close-packed planes (L12). The makeup of ordering-strengthened nanoclusters can
be tuned by heat treatments to affect deformation mechanisms. These findings
uncover a mixture of bonding preferences and their control at the nanoscopic
scale in CrCoNi and provide general opportunities for an atomistic-structure
study in concentrated alloys for the design of strong and ductile materials. | 2206.02004v1 |
2022-06-15 | Anomalous Ettingshausen effect in iron-carbon alloys | We have investigated the anomalous Ettingshausen effect (AEE) in iron-carbon
alloys, i.e., cast irons and steel, using the lock-in thermography. All the
alloys exhibit the clear AEE-induced temperature modulation, and their
anomalous Ettingshausen coefficient is an order of magnitude greater than that
of the pure iron at room temperature. The dimensionless figure of merit for AEE
in the ductile cast iron is 55 times greater than that in the pure iron owing
to the significant increase of the anomalous Ettingshausen coefficient. Our
result reveals a potential of iron-carbon alloys as transverse thermoelectric
materials, although the composition and microstructures optimizations are
necessary. | 2206.07322v2 |
2022-06-16 | Hardness prediction of age-hardening aluminum alloy based on ensemble learning | With the rapid development of artificial intelligence, the combination of
material database and machine learning has driven the progress of material
informatics. Because aluminum alloy is widely used in many fields, so it is
significant to predict the properties of aluminum alloy. In this thesis, the
data of Al-Cu-Mg-X (X: Zn, Zr, etc.) alloy are used to input the composition,
aging conditions (time and temperature) and predict its hardness. An ensemble
learning solution based on automatic machine learning and an attention
mechanism introduced into the secondary learner of deep neural network are
proposed respectively. The experimental results show that selecting the correct
secondary learner can further improve the prediction accuracy of the model.
This manuscript introduces the attention mechanism to improve the secondary
learner based on deep neural network, and obtains a fusion model with better
performance. The R-Square of the best model is 0.9697 and the MAE is 3.4518HV. | 2206.08011v1 |
2022-06-30 | Proximity spin-orbit coupling in graphene on alloyed transition metal dichalcogenides | The negligible intrinsic spin-orbit coupling (SOC) in graphene can be
enhanced by proximity effects in stacked heterostructures of graphene and
transition metal dichalcogenides (TMDCs). The composition of the TMDC layer
plays a key role in determining the nature and strength of the resultant SOC
induced in the graphene layer. Here, we study the evolution of the
proximity-induced SOC as the TMDC layer is deliberately defected. Alloyed ${\rm
G/W_{\chi}Mo_{1-\chi}Se_2}$ heterostructures with diverse compositions ($\chi$)
and defect distributions are simulated using density functional theory.
Comparison with continuum and tight-binding models allows both local and global
signatures of the metal-atom alloying to be clarified. Our findings show that,
despite some dramatic perturbation of local parameters for individual defects,
the low-energy spin and electronic behaviour follow a simple effective medium
model which depends only on the composition ratio of the metallic species in
the TMDC layer. Furthermore, we demonstrate that the topological state of such
alloyed systems can be feasibly tuned by controlling this ratio. | 2206.15313v1 |
2022-07-11 | Alloying 2D VSe2 with Pt: from a charge density wave state to a disordered insulator | We have analyzed by means of scanning tunneling microscopy and spectroscopy
the atomic and electronic structure of monolayers of 1T-VxPt1-xSe2 alloys grown
by molecular beam epitaxy on epitaxial graphene substrates. We have focused on
the composition range (0.1<x<0.35) where ferromagnetic behaviour has recently
been demonstrated. For low Pt concentration, (x=0.07 and x=0.21), small domains
(a few nanometres in diameter) exhibiting the characteristic superstructure of
the charge density wave (CDW) state of pristine VSe2 monolayer remain visible
on most of the sample surface. Thus alloying preserves the short range order of
the CDW phase, although it destroys its long range order. For higher Pt
concentration (x=0.35) a disordered alloy forms. It presents a fully developped
gap (a few tens meV in width) at the Fermi level and is thus a disordered
insulator. This gap exhibits strong variations at the nanometer scale,
reflecting the local fluctuations in the composition. An unexpectedly large
interaction of the TMD layer with the graphene substrate sets in for this
composition range. | 2207.04755v1 |
2022-08-02 | Combinatorial Discovery of Irradiation Damage Tolerant Nano-structured W-based alloys | One of the challenges in fusion reactors is the discovery of plasma facing
materials capable of withstanding extreme conditions, such as radiation damage
and high heat flux. Development of fusion materials can be a daunting task
since vast combinations of microstructures and compositions need to be
explored, each of which requires trial-and-error based irradiation experiments
and materials characterizations. Here, we utilize combinatorial experiments
that allow rapid and systematic characterizations of composition-microstructure
dependent irradiation damage behaviors of nanostructured tungsten alloys. The
combinatorial materials library of W-Re-Ta alloys was synthesized, followed by
the high-throughput experiments for probing irradiation damages to the
mechanical, thermal, and structural properties of the alloys. This highly
efficient technique allows rapid identification of composition ranges with
excellent damage tolerance. We find that the distribution of implanted He
clusters can be significantly altered by the addition of Ta and Re, which play
a critical role in determining property changes upon irradiation. | 2208.01271v1 |
2022-09-26 | Accurate ab initio modeling of solid solution strengthening in high entropy alloys | High entropy alloys (HEA) represent a class of materials with promising
properties, such as high strength and ductility, radiation damage tolerance,
etc. At the same time, a combinatorially large variety of compositions and a
complex structure render them quite hard to study using conventional methods.
In this work, we present a computationally efficient methodology based on ab
initio calculations within the coherent potential approximation. To make the
methodology predictive, we apply an exchange-correlation correction to the
equation of state and take into account thermal effects on the magnetic state
and the equilibrium volume. The approach shows good agreement with available
experimental data on bulk properties of solid solutions. As a particular case,
the workflow is applied to a series of iron-group HEA to investigate their
solid solution strengthening within a parameter-free model based on the
effective medium representation of an alloy. The results reveal intricate
interactions between alloy components, which we analyze by means of a simple
model of local bonding. Thanks to its computational efficiency, the methodology
can be used as a basis for an adaptive learning workflow for optimal design of
HEA. | 2209.12462v1 |
2022-10-12 | Experimental data management platform for data-driven investigation of combinatorial alloy thin films | Experimental materials data are heterogeneous and include a variety of
metadata for processing and characterization conditions, making the
implementation of data-driven approaches for developing novel materials
difficult. In this paper, we introduce the Thin-Film Alloy Database (TFADB), a
materials data management platform, designed for combinatorially investigated
thin-film alloys through various experimental tools. Using TFADB, researchers
can readily upload, edit, and retrieve multidimensional experimental alloy
data, such as composition, thickness, X-ray Diffraction, electrical
resistivity, nanoindentation, and image data. Furthermore,
composition-dependent properties from the database can easily be managed in a
format adequate to be preprocessed for machine learning analyses. High
flexibility of the software allows management of new types of materials data
that can be potentially acquired from new combinatorial experiments. | 2210.06027v1 |
2022-10-12 | Crystallographic ordering of Al and Sn in α-Ti | Increasing attention is being paid to $\alpha$$_2$ Ti$_3$(Al,Sn)
precipitation from the $\alpha$ phase of titanium alloys owing to its effect on
slip band formation, localisation and the implications for fatigue performance
in jet engine titanium. However, the early stages of $\alpha$$_2$ precipitation
have historically been difficult to observe in TEM, neutron diffraction or atom
probe analysis. Here, small angle X-ray scattering is used to reexamine the
phase boundary in binary Ti-Al and Ti-Sn alloys with around 500 ppmw O. It is
found that the phase boundaries in the literature are approximately correct, at
6.2 wt.% Al and 16.9 wt.% Sn, and that this favours the use of Al as a solid
solution strengthener over Sn for ambient temperature applications. However,
once O content and phase partitioning in $\alpha$+$\beta$ alloys are taken into
account, this implies that Al$_{eq}$ limits for future alloy design of critical
rotating parts should be lowered substantially. | 2210.06197v2 |
2022-10-28 | An innovative materials design protocol for the development of novel refractory high-entropy alloys for extreme environments | In the quest of new materials that can withstand severe irradiation and
mechanical extremes for advanced applications (e.g. fission reactors, fusion
devices, space applications, etc), design, prediction and control of advanced
materials beyond current material designs become a paramount goal. Here, though
a combined experimental and simulation methodology, the design of a new
nanocrystalline refractory high entropy alloy (RHEA) system is established.
Compositions of this alloy, assessed under extreme environments and in situ
electron-microscopy, revealed both high mechanical strength and thermal
stability, grain refinement under heavy ion irradiation and outstanding
irradiation resistance to dual-beam irradiation and helium implantation, marked
by remarkable resistance to defect generation, growth and coalescence. The
experimental and modeling results, which demonstrated notable agreement, can be
applied to design and rapidly assess other alloys subjected to extreme
environmental conditions. | 2210.16409v1 |
2022-11-19 | A new semiconducting perovskite alloy system made possible by gas-source molecular beam epitaxy | Optoelectronic technologies are based on families of semiconductor alloys. It
is rare that a new semiconductor alloy family is developed to the point where
epitaxial growth is possible; since the 1950s, this has happened approximately
once per decade. Here we demonstrate epitaxial thin film growth of
semiconducting chalcogenide perovskite alloys in the Ba-Zr-S-Se system by
gas-source molecular beam epitaxy (MBE). We stabilize the full range y = 0 ...
3 of compositions BaZrS$_{(3-y)}$Se$_y$ in the perovskite structure, up to and
including BaZrSe$_3$, by growing on BaZrS$_3$ epitaxial templates. The
resulting films are environmentally stable and the direct band gap ($E_g$)
varies strongly with Se content, as predicted by theory, covering the range
$E_g$ = 1.9 ... 1.4 eV for $y$ = 0 ... 3. This creates possibilities for
visible and near-infrared (VIS-NIR) optoelectronics, solid state lighting, and
solar cells using chalcogenide perovskites. | 2211.10787v2 |
2022-12-13 | Design guidelines for two-dimensional transition metal dichalcogenide alloys | Two-dimensional (2D) materials and Transition Metal Dichalcogenides (TMD) in
particular are at the forefront of nanotechnology. To tailor properties for
engineering applications, alloying strategies used for bulk metals in the last
century need to be extended to this novel class of materials. Here we present a
systematic analysis of the phase behaviour of substitutional 2D alloys in the
TMD family on both the metal and chalcogenide site. The phase behaviour is
quantified in terms of a metastability metric and benchmarked against
systematic computational screening of configurational energy landscapes. The
resulting Pettifor maps can be used to identify broad trends across chemical
spaces and as starting point for setting up rational search strategies in phase
space, thus allowing for targeted computational analysis of properties on
likely thermodynamically stable compounds. The results presented here also
constitute a useful guideline for synthesis of binary metal 2D TMDs alloys via
a range of synthesis techniques. | 2212.06597v1 |
2023-02-04 | An Interpretable Boosting-based Predictive Model for Transformation Temperatures of Shape Memory Alloys | In this study, we demonstrate how the incorporation of appropriate feature
engineering together with the selection of a Machine Learning (ML) algorithm
that best suits the available dataset, leads to the development of a predictive
model for transformation temperatures that can be applied to a wide range of
shape memory alloys. We develop a gradient boosting ML surrogate model capable
of predicting Martensite Start, Martensite Finish, Austenite Start, and
Austenite Finish transformation temperatures with an average accuracy of more
than 95% by explicitly taking care of potential distribution changes when
modeling different alloy systems. We included heat treatment, rolling,
extrusion processing parameters, and alloy system categorical features in the
model input features to achieve more accurate and realistic results. In
addition, using Shapley values, which are calculated based on the average
marginal contribution of features to all possible coalitions, this study was
able to gain insights into the governing features and their effect on predicted
transformation temperatures, providing a unique opportunity to examine the
critical parameters and features in martensite transformation temperatures. | 2302.02153v1 |
2023-03-01 | Molecular dynamics simulation of the transformation of Fe-Co alloy by machine learning force field based on atomic cluster expansion | The force field describing the calculated interaction between atoms or
molecules is the key to the accuracy of many molecular dynamics (MD) simulation
results. Compared with traditional or semi-empirical force fields, machine
learning force fields have the advantages of faster speed and higher precision.
We have employed the method of atomic cluster expansion (ACE) combined with
first-principles density functional theory (DFT) calculations for machine
learning, and successfully obtained the force field of the binary Fe-Co alloy.
Molecular dynamics simulations of Fe-Co alloy carried out using this ACE force
field predicted the correct phase transition range of Fe-Co alloy. | 2303.00753v1 |
2023-03-16 | Quasi-localized modes in crystalline high entropy alloys | High Entropy Alloys (HEAs) are designed by mixing multiple metallic species
in nearly the same amount to obtain crystalline or amorphous materials with
exceptional mechanical properties. Here we use molecular dynamics simulations
to investigate the role of positional and compositional disorder in determining
the low-frequency vibrational properties of CrMnFeCoNi HEAs. Our results show
that the expected dependence of the density of states on the frequency as
$D(\omega)\sim\omega^4$ is recovered for amorphous HEAs and is also observed
for partially crystallized alloys with deviations that depend on the degree of
crystallization. We find that the quasi-localized vibrations are still visible
in crystalline HEAs, albeit suppressed compared to the corresponding amorphous
alloys. Our work offers a unified perspective to describe HEA mechanical
properties in terms of their vibrational density of states. | 2303.09161v1 |
2023-04-09 | Multiscale modeling of kinetic sluggishness in equiatomic NiCoCr and NiCoCrFeMn single-phase solid solutions | Complex, concentrated, multi-component alloys have been shown to display
outstanding thermo-mechanical properties, that have been typically attributed
to sluggish diffusion, entropic, and lattice distortion effects. Here, we
investigate two metal alloys with such exemplary properties, the equiatomic,
single-phase, face-centered-cubic (FCC) alloys NiCoCr and NiCoCrFeMn, and we
compare their microstructural kinetics to the behaviors in a pure-Ni FCC metal.
We perform long-time, kinetic Monte Carlo (kMC) simulations, and we analyze in
detail the kinetics of atomic vacancies. We find that vacancies in both
concentrated alloys exhibit subdiffusive thermally driven dynamics, in direct
contrast to the diffusive dynamics of pure Ni. Subdiffusive dynamics shall be
attributed to dynamical sluggishness, that is modeled by a fractional Brownian
random walk. Furthermore, we analyze the statistics of waiting times, and we
interpret long power-law-distributed rest periods as a direct consequence of
barriers' energy-scales and lattice distortions. | 2304.04255v1 |
2023-04-13 | Energy landscape in NiCoCr-based middle-entropy alloys | NiCoCr middle-entropy alloy is known for its exceptional strength at both low
and elevated operating temperatures. Mechanical properties of NiCoCr-based
alloys are affected by certain features of the energy landscape, such as the
energy difference between the hcp and fcc phases (which is known to correlate
with the stacking fault energy in the fcc phase) and curvature of the energy
surface. We compute formation energies in the Ni-Co-Cr ternary and related
quaternary systems and investigate dependences of the relative energies on
composition. Such computed composition-structure-property relations can be
useful for tuning composition and designing next-generation alloys with
improved strength. | 2304.06852v1 |
2023-08-04 | Ambient Temperature Growth and Superconducting Properties of Ti-V Alloy Thin Films | A study on the optimization of ambient temperature growth and superconducting
properties of Ti-V alloy thin films grown on SiO2-coated Si substrate is
reported here. These films have been synthesized by co-sputtering of Ti and V
targets, and films having different Ti concentrations were deposited to get the
optimized critical temperature (TC) of thin films close to the bulk value. The
maximum TC of 5.2 K has been obtained in the Ti40V60 composition, which is
further increased to 6.2 K when a 10 nm thick Ti underlayer is added below the
Ti-V film. GIXRD measurements confirm the formation of Ti-V alloys in the
desired crystal structure. The upper critical field (HC2) of the thin films has
been estimated with the help of magnetotransport measurements. The utility of
Ti-V alloy thin films in superconducting radiation detection applications is
ascertained. | 2308.02303v1 |
2023-08-26 | Effect of Composition on Microstructural Evolution during Homogenization of 7XXX Alloys | The effect of composition on microstructure both at the length scale of the
secondary dendrite arm spacing and nano-sized dispersoids during homogenization
of Al-Zn-Cu-Mg-Zr alloys has been studied. A comprehensive model that can
predict the microstructure at both the length scales has been used for the
study. The microstructure predicted has been compared to that for two
homogenized samples from a directionally solidified AA7050 sample and a
reasonable match has been found. The initial as-cast microstructure for
different compositions is calculated using Scheil type solidification from
Thermo-Calc. The initial microstructure has a considerable influence on
microstructural evolution during homogenization. To take advantage of the
decreased solid solubility of Zr in {\alpha}-fcc, cooling rates during
solidification must be high enough to prevent precipitation of primary Al3Zr.
Under solidification with industrial cooling conditions solute rich alloys
leads to fewer dispersoids. Based on the study, an improved composition range
of 6-8%Zn, 1-2%Cu, 1-2%Mg and 0.1-0.15%Zr for 7XXX alloys has been proposed. | 2308.13799v1 |
2023-09-18 | Superconductivity in the bcc-type High-entropy Alloy TiHfNbTaMo | X-ray powder diffraction, electrical resistivity, magnetization, and
thermodynamic measurements were conducted to investigate the structure and
superconducting properties of TiHfNbTaMo, a novel high-entropy alloy possessing
a valence electron count (VEC) of 4.8. The TiHfNbTaMo HEA was discovered to
have a body-centered cubic structure and a microscopically homogeneous
distribution of the constituent elements. This material shows type-II
superconductivity with Tc = 3.42 K, lower critical field with 22.8 mT, and
upper critical field with 3.95 T. Low-temperature specific heat measurements
show that the alloy is a conventional s-wave type with a moderately coupled
superconductor. First-principles calculations show that the density of states
(DOS) of the TiHfNbTaMo alloy is dominated by hybrid d orbitals of these five
metal elements. Additionally, the TiHfNbTaMo HEA exhibits three van Hove
singularities. Furthermore, the VEC and the composition of the elements
(especially the Nb elemental content) affect the Tc of the bcc-type HEA. | 2309.09494v1 |
2023-09-19 | First-principles characterization of thermal conductivity in LaPO4-based alloys | Alloys based on lanthanum phosphate (LaPO$_{4}$) are often employed as
thermal barrier coatings, due to their low thermal conductivity and structural
stability over a wide temperature range. To enhance the thermal-insulation
performance of these alloys, it is essential to comprehensively understand the
fundamental physics governing their heat conduction. Here, we employ the Wigner
formulation of thermal transport in conjunction with first-principles
calculations to elucidate how the interplay between anharmonicity and
compositional disorder determines the thermal properties of
La$_x$Gd$_{1{-}x}$PO$_{4}$ alloys, and discuss the fundamental physics
underlying the emergence and coexistence of particle-like and wave-like
heat-transport mechanisms. Our predictions for microscopic vibrational
properties (temperature-dependent Raman spectrum) and for macroscopic thermal
conductivity are validated against experiments. Finally, we leverage these
findings to devise strategies to optimize the performance of thermal barrier
coatings. | 2309.10789v1 |
2023-09-21 | Composition-based phase stability model for multicomponent metal alloys | The vastness of the space of possible multicomponent metal alloys is hoped to
provide improved structural materials but also challenges traditional,
low-throughput materials design efforts. Computational screening could narrow
this search space if models for materials stability and desired properties
exist that are sufficiently inexpensive and accurate to efficiently guide
experiments. Towards this effort, here we develop a method to rapidly assess
the thermodynamic stability of a metal alloy composition of arbitrary number of
elements, stoichiometry, and temperature based on density functional theory
(DFT) data. In our model, the Gibbs free energy of the solid solution contains
binary enthalpy contributions and ideal configurational entropy, whereas only
enthalpy is considered for intermetallic competing phases. Compared to a past
model for predicting the formation of single-phase high-entropy alloys [Phys.
Rev. X 5, 011041 (2015)], our method is similarly inexpensive, since it
assesses enthalpies based on existing DFT data, but less heuristic, more
broadly applicable, and more accurate (70--75%) compared to experiment. | 2309.12222v1 |
2023-10-11 | Surface segregation in high-entropy alloys from alchemical machine learning | High-entropy alloys (HEAs), containing several metallic elements in
near-equimolar proportions, have long been of interest for their unique
mechanical properties. More recently, they have emerged as a promising platform
for the development of novel heterogeneous catalysts, because of the large
design space, and the synergistic effects between their components. In this
work we use a machine-learning potential that can model simultaneously up to 25
transition metals to study the tendency of different elements to segregate at
the surface of a HEA. We use as a starting point a potential that was
previously developed using exclusively crystalline bulk phases, and show that,
thanks to the physically-inspired functional form of the model, adding a much
smaller number of defective configurations makes it capable of describing
surface phenomena. We then present several computational studies of surface
segregation, including both a simulation of a 25-element alloy, that provides a
rough estimate of the relative surface propensity of the various elements, and
targeted studies of CoCrFeMnNi and IrFeCoNiCu, which provide further validation
of the model, and insights to guide the modeling and design of alloys for
heterogeneous catalysis. | 2310.07604v2 |
2023-10-11 | Revolutionising inverse design of magnesium alloys through generative adversarial networks | The utility of machine learning (ML) techniques in materials science has
accelerated materials design and discovery. However, the accuracy of ML models
- particularly deep neural networks - heavily relies on the quality and
quantity of the training data. Data collection methods often have limitations
arising from cost, difficulty, and resource-intensive human efforts. Thus,
limited high-quality data, especially for novel materials, poses a significant
challenge in developing reliable ML models. Generative adversarial networks
(GANs) offer one solution to augment datasets through synthetic sample
generation. The present work explores the application of GANs in magnesium (Mg)
alloy design, by training two deep neural networks within the structure of a
Wasserstein GAN to generate new (novel) alloys with desired mechanical
properties. This data augmentation-based strategy contributes to model
robustness, particularly in cases where traditional data collection is
impractical. The approach presented may expedite Mg alloy development, through
a GAN assisted inverse design approach. | 2310.07836v3 |
2023-10-26 | Local Coordination Modulates the Reflectivity of Liquefied Si-Ge Alloys | The properties of liquid Si-Ge binary systems at melting conditions deviate
from those expected by the ideal alloy approximation. Particularly, a
non-linear dependence of the dielectric functions occurs with the reflectivity
of liquid Si-Ge reaching a maximum at 50\% Ge content, being 10\% higher than
in pure Si or Ge. Using \textit{ab initio} methodologies, we modelled liquefied
Si-Ge alloys, unveiling very high coordination numbers and poor symmetry in the
first coordination shell with respect to Si and Ge, related to different
bonding properties. We simulated optical functions, quantitatively replicating
the aforementioned reflectivity trend and we highlighted a direct relationship
between atomic structure and optical properties, indicating that the unusual
optics arises from Si-Ge higher local coordination characterized by low
symmetry. We forecast further implications for the overall class of these
alloys. These findings expand our comprehension of liquefied semiconductors and
are essential for implementing controlled laser melting procedures to highly
dope these materials for advanced transistors, superconductors, sensors and
plasmonic devices. | 2310.17205v2 |
2023-10-27 | CELL: a Python package for cluster expansion with a focus on complex alloys | We present the Python package CELL, which provides a modular approach to the
cluster expansion (CE) method. CELL can treat a wide variety of substitutional
systems, including one-, two-, and three-dimensional alloys, in a general
multi-component and multi-sublattice framework. It is capable of dealing with
complex materials comprising several atoms in their parent lattice. CELL uses
state-of-the-art techniques for the construction of training data sets, model
selection, and finite-temperature simulations. The user interface consists of
well-documented Python classes and modules
(http://sol.physik.hu-berlin.de/cell/). CELL also provides visualization
utilities and can be interfaced with virtually any ab initio package,
total-energy codes based on interatomic potentials, and more. The usage and
capabilities of CELL are illustrated by a number of examples, comprising a
Cu-Pt surface alloy with oxygen adsorption, featuring two coupled binary
sublattices, and the thermodynamic analysis of its order-disorder transition;
the demixing transition and lattice-constant bowing of the Si-Ge alloy; and an
iterative CE approach for a complex clathrate compound with a parent lattice
consisting of 54 atoms. | 2310.18223v1 |
2023-11-04 | Effect of W alloying on the electronic structure, phase stability and thermoelectric properties of epitaxial CrN films | The effects of W alloying on the electronic structure, phase stability, and
thermoelectric properties of Cr1-xWxN films with 0 \leq x \leq 0.48 are
reported. Ab initio calculations indicate that dilute W alloying (x = 0.03)
results in flat bands from W 5d states and pushes the Fermi level EF into the
conduction band, while retaining dispersive Cr 3d bands. These features are
collectively conducive for both high electrical conductivity \sigma and high
Seebeck coefficient \alpha. Epitaxial Cr1-xWxN films grown on c-plane sapphire
by dc-magnetron sputtering show that \sigma increases with W additions of x
\leq 0.2. Higher W levels results in the formation of metallic Cr2N and W
precipitation, yielding high \sigma but low \alpha. These findings suggest that
restricting the W level to below its solubility limit in CrN is key to
realizing high thermoelectric properties in Cr1-xWxN alloys. | 2311.02453v1 |
2023-11-13 | The High-dimensional Phase Diagram and the Large CALPHAD Model | When alloy systems comprise more than three elements, the visualization of
the entire phase space becomes not only daunting but is also accompanied by a
data surge. Addressing this complexity, we delve into the FeNiCrMn alloy system
and introduce the Large CALPHAD Model (LCM). The LCM acts as a computational
conduit, capturing the entire phase space. Subsequently, this enormous data is
systematically structured using a high-dimensional phase diagram, aided by hash
tables and Depth-first Search (DFS), rendering it both digestible and
programmatically accessible. Remarkably, the LCM boasts a 97% classification
accuracy and a mean square error of 4.80*10-5 in phase volume prediction. Our
methodology successfully delineates 51 unique phase spaces in the FeNiCrMn
system, exemplifying its efficacy with the design of all 439 eutectic alloys.
This pioneering methodology signifies a monumental shift in alloy design
techniques or even multi-variable problems. | 2311.07174v1 |
2023-12-11 | Uncovering high-dimensional phase space and the application of Mixture of Experts (MoE) on building the Large CALPHAD Model (LCM) | This study presents a novel approach for analyzing and establishing Large
CALPHAD model (LCM) in complex alloy systems. Through the introduction of
"composition space volume", a multi-dimensional metric which allows to
quatitatively define alloy composition variations. Utilizing stochastic
methods, the study quantifies phase space complexity through phase density, and
model training costs through data density. This leads to a strategic
segmentation of the entire composition space, tailored to the complexity of
each segment, thereby reducing computational efforts in model training. A
significant advancement is the integration of segmented models using a Mixture
of Experts (MoE) approach, ensuring accurate portrayal of phase behaviors
across diverse composition spaces. This technique is demonstrated in
establishing a high-dimensional phase diagram for the FeCoNiTi system,
highlighting its efficiency and accuracy. The study's methodologies offer a
systematic and cost-effective framework for modeling complex alloy systems,
marking a step forward in the field of alloy design and analysis. | 2312.06429v1 |
2023-12-26 | Compositional Search of Stable Crystalline Structures in Multi-Component Alloys Using Generative Diffusion Models | Exploring the vast composition space of multi-component alloys presents a
challenging task for both \textit{ab initio} (first principles) and
experimental methods due to the time-consuming procedures involved. This
ultimately impedes the discovery of novel, stable materials that may display
exceptional properties. Here, the Crystal Diffusion Variational Autoencoder
(CDVAE) model is adapted to characterize the stable compositions of a well
studied multi-component alloy, NiFeCr, with two distinct crystalline phases
known to be stable across its compositional space. To this end, novel
extensions to CDVAE were proposed, enhancing the model's ability to reconstruct
configurations from their latent space within the test set by approximately
30\% . A fact that increases a model's probability of discovering new materials
when dealing with various crystalline structures. Afterwards, the new model is
applied for materials generation, demonstrating excellent agreement in
identifying stable configurations within the ternary phase space when compared
to first principles data. Finally, a computationally efficient framework for
inverse design is proposed, employing Molecular Dynamics (MD) simulations of
multi-component alloys with reliable interatomic potentials, enabling the
optimization of materials property across the phase space. | 2312.16073v1 |
2024-01-12 | Capturing short-range order in high-entropy alloys with machine learning potentials | Chemical short-range order (SRO) affects the distribution of elements
throughout the solid-solution phase of metallic alloys, thereby modifying the
background against which microstructural evolution occurs. Investigating such
chemistry-microstructure relationships requires atomistic models that act at
the appropriate length scales while capturing the intricacies of chemical bonds
leading to SRO. Here we consider various approaches for the construction of
training data sets for machine learning potentials (MLPs) for CrCoNi and
evaluate their performance in capturing SRO and its effects on materials
quantities of relevance for mechanical properties, such as stacking-fault
energy and phase stability. It is demonstrated that energy accuracy on test
sets often does not correlate with accuracy in capturing material properties,
which is fundamental in enabling large-scale atomistic simulations of metallic
alloys with high physical fidelity. Based on this analysis we systematically
derive design principles for the rational construction of MLPs that capture SRO
in the crystal and liquid phases of alloys. | 2401.06622v1 |
2024-01-15 | Fatigue Behavior of High-Entropy Alloys | High-entropy alloys (HEAs) refer to alloys composed of five or more elements
in equal or near-equal amounts or in an atomic concentration range of 5 to 35
atomic percent (at%). Different elemental ratios will affect the
microstructures of HEAs and provide them with unique properties. Based on past
research, HEAs have exhibited superior performance, relative to most
conventional alloys, with respect to many properties, such as strength,
toughness, corrosion resistance, magnetic behavior, etc. Among them, fatigue
behavior has been a topic of focus, due to its importance in industrial
applications. In this article, we summarized the research progress in the
HEA-fatigue behavior in the past ten years, including experimental results and
theoretical studies in subdivisions, such as high-cycle fatigue, low-cycle
fatigue, fatigue-crack growth, fatigue mechanisms, etc. The influence of the
processing and test methods on HEAs is described. The accuracy of several
commonly used prediction models is also outlined. Finally, unresolved issues
and suggestions on the direction of future research efforts are presented. | 2401.07418v1 |
2024-02-07 | Hot Carriers from Intra- and Interband Transitions in Gold-Silver Alloy Nanoparticles | Hot electrons and holes generated from the decay of localized surface
plasmons in metallic nanoparticles can be harnessed for applications in solar
energy conversion and sensing. In this paper, we study the generation of hot
carriers in large spherical gold-silver alloy nanoparticles using a recently
developed atomistic modelling approach that combines a solution of Maxwell's
equations with large-scale tight-binding simulations. We find that hot-carrier
properties depend sensitively on the alloy composition. Specifically,
nanoparticles with a large gold fraction produce hot carriers under visible
light illumination while nanoparticles with a large silver fraction require
higher photon energies to produce hot carriers. Moreover, most hot carriers in
nanoparticles with a large gold fraction originate from interband transitions
which give rise to energetic holes and "cold" electrons near the Fermi level.
Increasing the silver fraction enhances the generation rate of hot carriers
from intraband transitions which produce energetic electrons and "cold" holes.
These findings demonstrate that alloy composition is a powerful tuning
parameter for the design of nanoparticles for applications in solar energy
conversion and sensing that require precise control of hot-carrier properties. | 2402.05292v1 |
2024-02-26 | The influence of the phase and structural state on the low-temperature elastic properties of molybdenum-alloyed non-equiatomic high-entropy alloys of the Fe-Co-Ni-Cr system | The mechanical properties and microstructural evolution of a medium-entropy
alloy Co$_{17.5}$Cr$_{12.5}$Fe$_{55}$Ni$_{10}$Mo$_{5}$ (at%) in a low
temperature range (including the record low temperatures region down to 0.5 K)
were investigated. It has been established that low-temperature plastic
deformation initiates martensitic phase transformations in this alloy, and the
values of the dynamic modulus of elasticity correlate with the degree of phase
transformations. | 2402.17064v2 |
2024-02-29 | Mixed-halide perovskite alloys $\text{CsPb}(\text{I}_{1-x}^{}\text{Br}_x^{})_3^{}$ and $\text{CsPb}(\text{Br}_{1-x}^{}\text{Cl}_x^{})_3^{}$: New insight of configuration entropy effect from first principles and phase diagrams | Stability is one of the key issues in mixed-halide perovskite alloys which
are promising in emergent optoelectronics. Previous density-functional-theory
(DFT) and machine learning studies indicate that the formation-energy convex
hulls of these materials are very shallow, and stable alloy compositions are
rare. In this work, we revisit this problem using DFT with special focus on the
effects of configuration and vibration entropies. Allowed by the $20$-atomic
models for the $\text{CsPb}(\text{I}_{1-x}^{}\text{Br}_x^{})_3^{}$ and
$\text{CsPb}(\text{Br}_{1-x}^{}\text{Cl}_x^{})_3^{}$ series, the partition
functions and therewith thermodynamic state functions are calculated by
traversing all possible mixed-halide configurations. We can thus evaluate the
temperature- and system-dependent configuration entropy, which largely corrects
the conventional approach based on the ideal solution model. Finally,
temperature-composition phase diagrams that include $\alpha$, $\beta$, $\gamma$
and $\delta$ phases of both alloys are constructed based on the free energy
data, for which the contribution of phonon vibrations is included. | 2402.19274v1 |
2024-03-14 | A Processing Route to Chalcogenide Perovskites Alloys with Tunable Band Gap via Anion Exchange | We demonstrate synthesis of BaZr(S,Se)3 chalcogenide perovskite alloys by
selenization of BaZrS3 thin films. The anion-exchange process produces films
with tunable composition and band gap without changing the orthorhombic
perovskite crystal structure or the film microstructure. The direct band gap is
tunable between 1.5 and 1.9 eV. The alloy films made in this way feature 100x
stronger photoconductive response and a lower density of extended defects,
compared to alloy films made by direct growth. The perovskite structure is
stable in high-selenium-content thin films with and without epitaxy. The
manufacturing-compatible process of selenization in H2Se gas may spur the
development of chalcogenide perovskite solar cell technology. | 2403.09016v1 |
2024-04-11 | Point defects in CdTe and CdTeSe alloy: a first principles investigation with DFT+U | CdTe and its alloy CdTeSe are widely used in optoelectronic devices, such as
radiation detectors and solar cells, due to their superior electrical
properties. However, the formation of defects and defect complexes in these
materials can significantly affect their performance. As a result,
understanding the defect formation and recombination processes in CdTe and
CdTeSe alloy is of great importance. In recent years, density functional theory
(DFT) calculations have emerged as a powerful tool for investigating the
properties of defects in semiconductors. In this paper, we use DFT+U
calculations to comprehensively study the properties of intrinsic defects as
well as extrinsic defects induced by commonly used dopants, such as Cu and
group V elements, in CdTe and CdTeSe alloy. This work provides insights into
the effects of these defects on the electrical and optical properties of the
material. | 2404.07796v1 |
2013-07-29 | Effect of surface hydrogen on the anomalous surface segregation behavior of Cr in Fe-rich Fe-Cr alloys | The segregation behavior of Cr in dilute Fe-Cr alloys is known to be
anomalous since the main barrier for surface segregation of Cr in these alloys
arises not from the topmost surface layer but from the subsurface layer where
the solution energy of Cr is much more endothermic as compared to the topmost
surface layer. The Fe-Cr alloys are candidate structural materials for the new
generation of nuclear reactors. The surfaces of these alloys will be exposed to
hydrogen or its isotopes in these reactors, and although hydrogen is soluble
neither in Fe nor in Fe-Cr alloys, it is known that the adsorption energy of
hydrogen on the surface of iron is not only exothermic but relatively large.
This clearly raises the question of the effect of the hydrogen adsorbed on the
surface of iron on the segregation behavior of chromium towards the surface of
iron. In this paper we show, on the basis of our ab initio density functional
theory calculations, that the presence of hydrogen on the surface of iron leads
to a considerably reduced barrier for Cr segregation to both the topmost
surface layer and the subsurface layer, but the subsurface layer still controls
the barrier for surface segregation. This reduction in the barrier for surface
segregation is due to the nature of the Cr-H couple that acts in a complex and
synergistic manner. The presence of Cr enhances the exothermic nature of
hydrogen adsorption that in turn leads to a reduced barrier for surface
segregation. These results should be included in the multiscale modeling of
Fe-Cr alloys. | 1307.7588v1 |
2017-01-11 | Using transmission Kikuchi diffraction to characterise α variants in an α + β titanium alloy | Two phase titanium alloys are important for high performance engineering
components, such as aeroengine discs. The microstructures of these alloys are
tailored during thermomechanical processing to precisely control phase
factions, morphology and crystallographic orientations. In bimodal two phase
({\alpha} + {\beta}) Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloys there are often three
microstructural lengthscales to consider: large (~10 {\mu}m) equiaxed primary
{\alpha}; >200 nm thick plate {\alpha} with a basketweave morphology; and very
fine scaled (>50 nm plate thickness) secondary {\alpha} that grows between the
larger {\alpha} plates surrounded by retained {\beta}. In this work, we utilise
high spatial resolution transmission Kikuchi diffraction (TKD, also known as
transmission based electron backscatter diffraction, t-EBSD) and scanning
electron microscopy (SEM) based forward scattering electron imaging to resolve
the structures and orientations of basketweave and secondary {\alpha} in
Ti-6242. We analyse the {\alpha} variants formed within one prior {\beta}
grain, and test whether existing theories of habit planes of the phase
transformation are upheld. Our analysis is important in understanding both the
thermomechanical processing strategy of new bimodal two-phase titanium alloys,
as well the ultimate performance of these alloys in complex loading regimes
such as dwell fatigue. Our paper champions the significant increase in spatial
resolution afforded using transmission techniques, combined with the ease of
SEM based analysis using conventional electron backscatter diffraction (EBSD)
systems and forescatter detector (FSD) imaging, to study the nanostructure of
real-world engineering alloys. | 1701.03014v2 |
2017-03-31 | Evolution of Raman spectra in Mo$_{1-x}$W$_x$Te$_2$ alloys | The structural polymorphism in transition metal dichalcogenides (TMDs)
provides exciting opportunities for developing advanced electronics. For
example, MoTe$_2$ crystallizes in the 2H semiconducting phase at ambient
temperature and pressure, but transitions into the 1T$^\prime$ semimetallic
phase at high temperatures. Alloying MoTe$_2$ with WTe$_2$ reduces the energy
barrier between these two phases, while also allowing access to the T$_d$ Weyl
semimetal phase. The MoWTe$_2$ alloy system is therefore promising for
developing phase change memory technology. However, achieving this goal
necessitates a detailed understanding of the phase composition in the
MoTe$_2$-WTe$_2$ system. We combine polarization-resolved Raman spectroscopy
with X-ray diffraction (XRD) and scanning transmission electron microscopy
(STEM) to study MoWTe$_2$ alloys over the full compositional range x from 0 to
1. We identify Raman and XRD signatures characteristic of the 2H, 1T$^\prime$,
and T$_d$ structural phases that agree with density-functional theory (DFT)
calculations, and use them to identify phase fields in the MoTe$_2$-WTe$_2$
system, including single-phase 2H, 1T$^\prime$, and T$_d$ regions, as well as a
two-phase 1T$^\prime$ + T$_d$ region. Disorder arising from compositional
fluctuations in MoWTe$_2$ alloys breaks inversion and translational symmetry,
leading to the activation of an infrared 1T$^\prime$-MoTe$_2$ mode and the
enhancement of a double-resonance Raman process in 2H-MoWTe$_2$ alloys.
Compositional fluctuations limit the phonon correlation length, which we
estimate by fitting the observed asymmetric Raman lineshapes with a phonon
confinement model. These observations reveal the important role of disorder in
MoWTe$_2$ alloys, clarify the structural phase boundaries, and provide a
foundation for future explorations of phase transitions and electronic
phenomena in this system. | 1703.10985v1 |
2018-11-21 | Beneficial influence of Hf and Zr additions to Nb4at.%Ta on the vortex pinning of Nb$_{3}$Sn with and without an O source | Here we show that addition of Hf to Nb4Ta can significantly improve the high
field performance of Nb$_{3}$Sn, making it suitable for dipole magnets for
Future Circular Collider (FCC). A big challenge for the FCC is that a realistic
production target for FCC Nb3Sn requires ~30% improvement over current
conductor performance. Recent success with internal oxidation(IO) of Nb-Zr
precursor has shown significant improvement in the layer J$_{c}$ of Nb$_{3}$Sn
wires, albeit the complication of providing an internal O$_{2}$ diffusion path
and avoiding degradation of irreversibility field($_{irr}$). We compare Zr and
Hf additions to the standard Nb4Ta alloy of maximum H$_{c2}$ and H$_{irr}$.
Nb4Ta rods with 1Zr or 1Hf were made into monofilament wires with and without
SnO$_{2}$ and their properties measured over the entire superconducting range
up to 31 T. We found that group IV alloying of Nb4Ta raises H$_{irr}$, though
adding O$_{2}$ still degrades this slightly. As noted in Nb1Zr studies, the
pinning force density F$_{p}$ is strongly enhanced and its peak value shifted
to higher field by IO. A surprising result of this work is that we found better
properties in Nb4Ta1Hf without SnO$_{2}$, F$_{pmax}$ achieving 2.35 times that
of the standard Nb4Ta alloy, while the oxidized Nb4Ta1Zr alloy achieved 1.54
times that of the Nb4Ta alloy. The highest layer J$_{c}$ (16 T, 4.2 K) of 3700
A/mm$^{2}$ was found in the SnO$_{2}$-free wire made with Nb4Ta1Hf alloy. Using
a standard A15 cross-section fraction of 60% for modern PIT and RRP wires, we
estimated that a non-Cu J$_{c}$ of 2200 A/mm$^{2}$ is obtainable in modern
conductors, well above the 1500A/mm$^{2}$ FCC specification. Moreover, the best
properties were obtained without SnO$_{2}$, the Nb4Ta1Hf alloy appears to open
a straightforward route to enhanced properties in Nb$_{3}$Sn wires. | 1811.08867v1 |
2022-03-06 | Hall coefficient in amorphous alloys: critical behavior and quantitative test of quantum corrections due to weak localization and electron-electron interactions | Here, we present the measurements of $R_H$ in a series of $Ti_xSi_{100-x}$
amorphous reaching the critical concentration, $x_c\approx9-9.5$. For
$x\geq17$, the Hall coefficient displays the behavior predicted by the
perturbation theory, $R_H^{-1}\left(T\right)=R_H^{-1}\left(0\right)+bT^{1/2}$,
which extends up to the temperature 150 K. The temperature dependence gets
stronger in alloys with lower $x$; $R_H\left(0\right)$ diverges at $x_c$
displaying critical behavior. We used the combined conductivity and Hall
coefficient data for alloys with high Ti content to test the theories of
quantum corrections to conductivity. We found that the correction due to weak
localization is dominated by the electron-phonon scattering with the rate
varying with temperature as $\tau_{ep}^{-1}=A_{ep}T^2$. The extracted parameter
$A_{ep}$ is in good agreement with the theory that considers the incomplete
drag of impurities by lattice vibrations. The spin-orbit scattering time
extracted from the weak localization correction was found to be two orders of
magnitude larger than the time given by the standard estimate
$\tau_{so}\approx\tau\left(\hbar c/e^2Z\right)^4$. The theory of the EEI
quantum correction was tested using the Hall coefficient and specific heat data
for Ti-Si and $\left(Ag_{0.5}Cu_{0.5}\right)_{100-x}Ge_x$ amorphous alloys,
which allowed us to estimate all microscopic parameters needed by the theory.
We found that, within the accuracy of our measurements, the EEI theory works
exactly for alloys that follow the free electron model
[$\left(Ag_{0.5}Cu_{0.5}\right)_{100-x}Ge_x$ with $x\le50$.] The deviation from
the theory observed in all Ti-Si alloys and in Ag-Cu-Ge alloys with $x\geq60$
can be qualitatively explained by weakening of the electron screening in the
systems. | 2203.03029v1 |
2021-09-01 | Electronic and optical properties of Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ alloys lattice-matched to Ge | We present a combined experimental and theoretical analysis of the evolution
of the near-band gap electronic and optical properties of
Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ alloys lattice-matched to Ge and GaAs substrates.
We perform photoreflectance (PR) and photoluminescence (PL) measurements on
Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ epitaxial layers grown via chemical vapour
deposition, for Si (Sn) compositions up to $x =$ 9.6% ($y =$ 2.5%). Our
measurements indicate the presence of an indirect fundamental band gap, with PL
observed $\approx$ 200-250 meV lower in energy than the direct $E_0$ transition
identified by PR measurements. The measured PL is Ge-like, suggesting that the
alloy conduction band (CB) edge is primarily derived from the Ge L-point CB
minimum. Interpretation of the PR and PL measurements is supported by atomistic
electronic structure calculations. Effective alloy band structures calculated
via density functional theory confirm the presence of an indirect fundamental
band gap, and reveal the origin of the observed inhomogeneous broadening of the
measured optical spectra as being alloy-induced band hybridisation occurring
close in energy to the CB edge. To analyze the evolution of the band gap,
semi-empirical tight-binding (TB) calculations are employed to enable
calculations for large supercell sizes. TB calculations reveal that the alloy
CB edge is hybridized in nature, consisting at low Si and Sn compositions of an
admixture of Ge L-, $\Gamma$- and X-point CB edge states, and confirm that the
alloy CB edge retains primarily Ge L-point CB edge character. Our experimental
measurements and theoretical calculations confirm a direct transition energy
close to 1 eV in magnitude for Si and Sn compositions $x =$ 6.8 - 9.6% and $y
=$ 1.6 - 2.2%. | 2109.02782v3 |
2021-09-29 | Thermal evolution of nanocrystalline co-sputtered Ni-Zr alloy films: Structural, magnetic and MD simulation studies | Monophasic and homogeneous Ni10Zr7 nanocrystalline alloy films were
successfully grown at room temperature by co-sputtering in an indigenously
developed three-gun DC/RF magnetron sputtering unit. The films could be
produced with long-range crystallographic and chemical order in the alloy, thus
overcoming the widely acknowledged inherent proclivity of the glass forming
Ni-Zr couple towards amorphization. Crystallinity of these alloys is a
desirable feature with regard to improved efficacy in applications such as
hydrogen storage, catalytic activity and nuclear reactor engineering, to name a
few. Thermal stability of this crystalline phase, being vital for transition to
viable applications, was investigated through systematic annealing of the alloy
films at 473 K, 673 K and 923 K for various durations. While the films were
stable at 473 K, the effect of annealing at 673 K was to create segregation
into nanocrystalline Ni (superparamagnetic) and amorphous Ni+Zr (non-magnetic)
phases. Detailed analyses of the physical and magnetic structures before and
after annealing were performed through several techniques effectual in
analyzing stratified configurations and the findings were all consistent with
each other. Polarized neutron and X-ray reflectometry, grazing incidence x-ray
diffraction, time-of-flight secondary ion mass spectroscopy and X-ray
photoelectron spectroscopy were used to gauge phase separation at nanometer
length scales. SQUID based magnetometry was used to investigate macroscopic
magnetic properties. Simulated annealing performed on this system using
molecular dynamic calculations corroborated well with the experimental results.
This study provides a thorough understanding of the creation and thermal
evolution of a crystalline Ni-Zr alloy. | 2109.14228v1 |
2022-07-06 | Increasing the mobility and power-electronics figure of merit of AlGaN with atomically thin AlN/GaN digital-alloy superlattices | Alloy scattering in random AlGaN alloys drastically reduces the electron
mobility and therefore the power-electronics figure of merit. As a result, Al
compositions greater than 75% are required to obtain even a two-fold increase
of the Baliga figure of merit compared to GaN. However, beyond approximately
80% Al composition, donors in AlGaN undergo the DX transition which makes
impurity doping increasingly more difficult. Moreover, the contact resistance
increases exponentially with increasing Al content, and integration with
dielectrics becomes difficult due to the upward shift of the conduction band.
Atomically thin superlattices of AlN and GaN, also known as digital alloys, are
known to grow experimentally under appropriate growth conditions. These
chemically ordered nanostructures could offer significantly enhanced figure of
merit compared to their random-alloy counterparts due to the absence of alloy
scattering, as well as better integration with contact metals and dielectrics.
In this work, we investigate the electronic structure and phonon-limited
electron mobility of atomically thin AlN/GaN digital-alloy superlattices using
first-principles calculations based on density-functional and many-body
perturbation theory. The band gap of the atomically thin superlattices reaches
4.8 eV, and the in-plane (out-of-plane) mobility is 369 (452) cm$^2$ V$^{-1}$
s$^{-1}$. Using the modified Baliga figure of merit that accounts for the
dopant ionization energy, we demonstrate that atomically thin AlN/GaN
superlattices with a monolayer sublattice periodicity have the highest modified
Baliga figure of merit among several technologically relevant ultra-wide
band-gap materials, including random AlGaN, $\beta$-Ga$_{2}$O$_{3}$, cBN, and
diamond. | 2207.02809v1 |
2022-08-24 | In-situ neutron diffraction during reversible deuterium loading in Ti-rich and Mn-substituted Ti(Fe,Mn)0.90 alloys | Hydrogen is an efficient energy carrier that can be produced from renewable
sources, enabling the transition towards CO2-free energy. Hydrogen can be
stored for a long period in the solid-state, with suitable alloys. Ti-rich
TiFe0.90 compound exhibits a mild activation process for the first
hydrogenation, and Ti(Fe,Mn)0.90 substituted alloys can lead to the fine tuning
of equilibrium pressure as a function of the final application. In this study,
the crystal structure of TiFe(0.90-x)Mnx alloys (x = 0, 0.05 and 0.10) and
their deuterides has been determined by in-situ neutron diffraction, while
recording Pressure-Composition Isotherms at room temperature. The investigation
aims at analysing the influence of Mn for Fe substitution in Ti-rich
Ti(Fe,Mn)0.90 alloys on structural properties during reversible deuterium
loading, which is still unsolved and seldom explored. After activation, samples
have been transferred into custom-made stainless-steel and aluminium alloy
cells used for in-situ neutron diffraction experiments during deuterium loading
at ILL and ISIS neutron facilities, respectively. The study enables remarkable
understanding on hydrogen storage, basic structural knowledge, and support to
the industrial application of TiFe-type alloys for integrated hydrogen tank in
energy storage systems by determining the volume expansion during deuteration.
Furthermore, the study demonstrates that different contents of Mn do not
significantly change the volumetric expansion during phase transitions,
affecting only the deuterium content for the {\gamma} phase and the cell
evolution for the \b{eta} phase. The study confirms that the deuterated
structures of the {\gamma} phase upon absorption, \b{eta} and {\alpha} phase
upon desorption, correspond to S.G. Cmmm, P2221 and Pm-3m, respectively. | 2208.11526v2 |
2022-09-19 | Effect of alloying on the microstructure, phase stability, hardness and partitioning behavior of a new dual-superlattice nickel-based superalloy | A novel y-y'-y" dual-superlattice superalloy, with promising mechanical
properties up to elevated temperatures was recently reported. The present work
employs state of the art chemical and spatial characterization techniques to
study the effect systematic additions of Mo, W and Fe and variations in Nb and
Al contents have on the phase fraction, thermal stability, elemental
partitioning and mechanical properties. Alloys were produced through arc
melting followed by heat treatment. Multi-scale characterization techniques and
hardness testing were employed to characterize their microstructure, thermal
stability and mechanical properties. Alterations in such properties or in
elemental partitioning behaviour were then explained through thermodynamic
modelling.
A modest addition of 1.8 at.% Mo had a strong effect on the microstructure
and thermal stability: it minimized microstructural coarsening during heat
treatments while not significantly decreasing the y' solvus temperature. A
reduction of Nb by 0.6 at.%, strongly reduced the y" volume fraction, without
affecting the y' volume fraction. The reduced precipitate fraction led to a
significant reduction in alloy hardness. Fe, added to achieve better
processability and reduced material cost, decreased the y' solvus temperature
and caused rapid microstructural coarsening during heat treatments, without
affecting alloy hardness. A reduction of Al by 0.4 at.%, reduced the y' volume
fraction and the y' solvus temperature, also without affecting alloy hardness.
The addition of 0.9 at.% W decreased the y' solvus temperature but increased
both precipitate volume fractions. These data will be invaluable to optimize
current alloy design and to inform future alloy design efforts. | 2209.08948v1 |
2023-11-23 | 3D microstructure characterization of Cu 25Cr solid state sintered alloy using X-ray computed tomography and machine learning assisted segmentation | Cu-Cr-based alloys with Cr content from 5 to 50 wt.% are widely used as
electrical contacts for vacuum interrupters for medium voltage applications
because of their excellent combination of mechanical, thermal, and electrical
conductivity. Cu-Cr electrical contacts are usually processed by sintering or
casting processes. For solid-state sintered Cu-Cr materials, the physical
properties vary as a function of the Cr content, phase morphology and porosity
volume fraction. Some studies have investigated the effect of the
microstructural characteristics of Cu-Cr alloys with different Cr content and
morphology on their properties. However, the porosity characterization and Cr
spatial distribution and how they affect these alloys' physical properties are
not as well documented. In this study, we report an in-depth 3D
characterization of the porosity and Cr-phase of solid-state sintered Cu-25Cr
alloys with three final relative densities using X-ray Computed Tomography
(XCT). An image analysis algorithm assisted by a machine learning-based
segmentation method has been specifically developed. Results show that for
Cu-25Cr solid sintered alloys there are mainly two types of pores, pores
located at the Cu/Cr interfaces, and pores within the Cu matrix. The
interfacial porosity represents the larger volume fraction, over 75% of the
total porosity for all cases, forming a large network of interconnected pores.
With the increase of final density, the Cu-matrix becomes nearly fully dense
while interfacial pores still represent the largest fraction decreases in size
and volume. These interfacial pores networks are believed to be formed due to
poor filling and packing of Cu around the percolated Cr-phase. These
observations might be helpful to optimize the functional properties of Cu-Cr
sintered alloys. | 2311.13904v1 |
2024-02-24 | Prediction of novel ordered phases in U-X (X= Zr, Sc, Ti, V, Cr, Y, Nb, Mo, Hf, Ta, W) binary alloys under high pressure | U-based binary alloys have been widely adopted in fast nuclear reactors, but
their stability under extreme conditions of high-pressure is almost unknown,
mounting up to latent risk in applications. Here, possible ordered phases in
U-Zr system up to 200 GPa are comprehensively investigated by unbiased
first-principles structure prediction. Stable U2Zr, metastable U3Zr and U4Zr
phases are discovered for the first time, which exhibit strong stability under
compression. They all are metallic, with 5f electrons of uranium dominating the
electronic density of states near the Fermi level. Prominent ionic interactions
between U and Zr atoms, as well as covalent interactions between adjacent
uranium atoms, are found. The same strategy is applied to explore the stability
of ordered phases in other U-based binary transition metal alloys, U-X (X= Sc,
Ti, V, Cr, Y, Nb, Mo, Hf, Ta, W). Stable and metastable ordered phases similar
to U-Zr alloy are unveiled, all with similar electronic structures. For these
alloys, we find that the structure of U2X (X=Zr, Ti, Hf) hosts a unique hybrid
phase transition similar to U2Nb, which is a superposition of a first-order
transition and a second-order transition. The prediction of these novel phases
not only refutes the stability of the long-believed ordered phase I4/mmm-U2Mo,
but also rewrites the phase diagrams of U-X (X= Zr, Sc, Ti, V, Cr, Nb, Mo, Hf,
Ta) alloys under high pressure. All of these findings promote our understanding
of the high-pressure behavior of the broad category of U-based binary alloys
with transition metals. | 2402.15793v1 |
1995-11-03 | Calculation of electronic properties of amorphous alloys | We describe the application of the
locally-self-consistent-multiple-scattering (LSMS)[1] method to amorphous
alloys. The LSMS algorithm is optimized for the Intel XP/S-150, a
multiple-instruction-multiple-data parallel computer with 1024 nodes and 2
compute processors per node. The electron density at each site is determined by
solving the multiple scattering equation for atoms within a specified distance
of the atom under consideration. Because this method is carried out in real
space it is ideal for treating amorphous alloys. We have adapted the code to
the calculation of the electronic properties of amorphous alloys. In these
calculations we determine the potentials in the atomic sphere approximation
self consistently at each site, unlike previous calculations[2] where we
determined the potentials self consistently at an average site. With these
self-consistent potentials, we then calculate electronic properties of various
amorphous alloy systems. We present calculated total electronic densities of
states for amorphous Ni$_{80}$P$_{20}$ and Ni$_{40}$Pd$_{40}$P$_{20}$ with 300
atoms in a supercell. | 9511021v1 |
1999-11-17 | On Coherency-Induced Ordering in Substitutional Alloys: I. Analytical | As pointed out by Linus Pauling in his classic work on the relationship
between crystal packing and ionic radius ratio, a difference in atomic size can
be accommodated more readily by an ordered structure than by a disordered one.
Because of mathematical complexity, however, very few works have been reported
for substitutional alloys. In this work, coherency-induced ordering in
substitutional alloys is examined through a simple model based on a
two-dimensional square lattice. Within the assumption of nearest neighbor
interactions on a square lattice, both modified Bragg-Williams and Onsager
approaches show that coherency strain arising due to atomic mismatch can exert
profound effects on order-disorder transitions in substitutional alloys. If the
alloy system is elastically homogeneous and Vegard's law is obeyed, the
order-disorder transition is of a second-order kinetics. If the atomic
mismatches significantly deviate from Vegard's law, however, the transition may
become a first-order kinetics, as the configurational free energy surface is
composed of double wells. At the transition of a first-order kinetics, the
lattice parameter can either increase or decrease upon heating, i.e., the
lattice parameter of an ordered state can be less or greater than that of a
disordered state. The results of Onsager's approach are independently confirmed
with those of the Discrete Atom Method, a Monte Carlo technique predicated upon
the combination of statistical mechanics and linear elasticity. | 9911267v2 |
2000-09-05 | BAs and boride III-V alloys | Boron arsenide, the typically-ignored member of the III-V arsenide series
BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma
conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an
X_1c-like indirect band gap, and its bond charge is distributed almost equally
on the two atoms in the unit cell, exhibiting nearly perfect covalency. The
reasons for these are tracked down to the anomalously low atomic p orbital
energy in the boron and to the unusually strong s-s repulsion in BAs relative
to most other III-V compounds. We find unexpected valence band offsets of BAs
with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is
significantly higher than that of AlAs, despite the much smaller bond length of
BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects
result from the unusually strong mixing of the cation and anion states at the
VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and
composition-independent band gap bowing. This means that while addition of
small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of
boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the
conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing
enthalpies which are smaller than in GaN-GaAs alloys. The unique features of
boride III-V alloys offer new opportunities in band gap engineering. | 0009063v1 |
2002-02-17 | Theoretical Study of Magnetism and Superconductivity in 3d Transition-Metal- MgB_2 Alloys | We have studied the electronic structure of 3d transition-metal- MgB_{2}
alloys, Mg_{0.97}TM_{0.03}B_{2}, (TM\equiv Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn) using KKR-CPA method in the atomic-sphere approximation. For unpolarized
calculations, our results for Mg_{0.97}TM_{0.03}B_{2} alloys are similar to
that of 3d impurities in other s and s-p metals. In particular, the local
densities of states (DOS) associated with the 3d impurities are similar to our
earlier work on 3d impurities in bulk Al. For spin-polarized calculations, we
find only the alloys of V, Cr, Mn, Fe and Co with MgB_{2} to be magnetic of all
the 3d elements. We also find that Cr and Mn in MgB_{2} have a relatively large
local magnetic moment of 2.43 \mu_{B} and 2.87 \mu_{B}, respectively. We have
used the unpolarized, self-consistent potentials of Mg_{0.97}TM_{0.03}B_{2}
alloys, obtained within the coherent- potential approximation, to calculate the
electron-phonon coupling constant \lambda using the Gaspari-Georffy formalism
and the superconducting transition temperature T_{c} using the Allen-Dynes
equation. We find that the calculated T_{c} is the lowest for
Mg_{0.97}Cr_{0.03}B_{2} and the highest for Mg_{0.97}Zn_{0.03}B_{2}, in
qualitative agreement with experiment. The calculated trend in variation of
T_{c} from Mn to Zn is also similar to the available experimental data. Our
analysis of the variation in T_{c}, in terms of the DOS and the spectral
function along \Gamma to A direction, shows the variation to be an interplay
between the total DOS at the Fermi energy and the creation/removal of states
along \Gamma to A direction [P. P. Singh, cond-mat/0201093]. | 0202285v1 |
2004-03-19 | Influence of intermartensitic transitions on transport properties of Ni2.16Mn0.84Ga alloy | Magnetic, transport, and x-ray diffraction measurements of ferromagnetic
shape memory alloy Ni$_{2.16}$Mn$_{0.84}$Ga revealed that this alloy undergoes
an intermartensitic transition upon cooling, whereas no such a transition is
observed upon subsequent heating. The difference in the modulation of the
martensite forming upon cooling from the high-temperature austenitic state
[5-layered (5M) martensite], and the martensite forming upon the
intermartensitic transition [7-layered (7M) martensite] strongly affects the
magnetic and transport properties of the alloy and results in a large thermal
hysteresis of the resistivity $\rho$ and magnetization $M$. The
intermartensitic transition has an especially marked influence on the transport
properties, as is evident from a large difference in the resistivity of the 5M
and 7M martensite, $(\rho_{\mathrm{5M}} - \rho_{\mathrm{7M}})/\rho
_{\mathrm{5M}} \approx 15%$, which is larger than the jump of resistivity at
the martensitic transition from the cubic austenitic phase to the monoclinic 5M
martensitic phase. We assume that this significant difference in $\rho$ between
the martensitic phases is accounted for by nesting features of the Fermi
surface. It is also suggested that the nesting hypothesis can explain the
uncommon behavior of the resistivity at the martensitic transition, observed in
stoichiometric and near-stoichiometric Ni-Mn-Ga alloys. | 0403495v1 |
2005-06-17 | Interface-dominated Growth of a Metastable Novel Alloy Phase | A new \textit{D0$_{23}$} metastable phase of Cu$_3$Au is found to grow at the
interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent
of formation of this novel alloy phase depends upon an optimal range of
interfacial width primarily governed by the deposition wattage of the
dc-magnetron used. Such interfacially confined growth is utilized to grow a
$\sim$ 300 nm thick Au/Cu multilayer with thickness of each layer nearly equal
to the optimal interfacial width which was obtained from secondary ion mass
spectrometry (SIMS) data. This growth technique is observed to enhance the
formation of the novel alloy phase to a considerable extent. SIMS depth profile
also indicates that the mass fragment corresponding to Cu$_3$Au occupies the
whole film while x-ray diffraction (XRD) shows almost all the strong peaks
belonging to the \textit{D0$_{23}$} structure. High resolution cross-sectional
transmission electron microscopy (HR-XTEM) shows the near perfect growth of the
individual layers and also the lattice image of the alloy phase in the
interfacial region. Vacuum annealing of the alloy film and XRD studies indicate
stabilization of the \textit{D0$_{23}$} phase at $\sim$ 150$^{\circ}$C. The
role of interfacial confinement, the interplay between interfacial strain and
free energy and the hyperthermal species generated during the sputtering
process are discussed. | 0506427v1 |
2005-07-14 | Superstructure in nano-crystalline Al50Cu28Fe22 alloy | This work reports the formation of nano- crystalline Al50Cu28Fe22 by
high-energy milling. For obtaining the nano-crystalline material, the
Al50Cu28Fe22 alloy synthesized through slow cooling of the molten alloy was
subjected to ball milling, which was carried out in attritor mill at 400 rpm
for 5 h, 10 h, 20 h, 40 h and 80 h with a ball to powder ratio 40 : 1 in hexane
medium. The x-ray diffraction observation of ball-milled samples revealed that
the milling duration of $5h$ to $40 hrs$ has led to the formation of
nano-phase. The average crystallite size comprising the nano-phase has been
found to be $\sim 17 nm$. When the nano-crystalline alloy, Al50Cu28Fe22 was
vacuum annealed at a temperature of 500$^0C$ for 5 to 20 hrs, new structural
phases representing superstructures of the parent nano-crystalline phase were
found . The superstructure have been found to correspond to simple cubic with
$a = \sqrt 2a_p$ and face central cubic with a = 2a_p (a_p = lattce parameter
of parent nano-crystalline alloy). It has been proposed that the formation of
different type of superstructure resulting due to different duration of ball
milling followed by annealing is possibly governed by minimization of free
energy of the disordered B2 phase. | 0507343v1 |
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